My fw version is v3.4.6 PX4fmu-v2,if the develop code is that Branch Copter v3.4.6 in github https://github.com/ArduPilot/ardupilot
Do you really need that code version to modify? If you were going to stay with version 3 for some special reason, and modify it, then at least use 3.6.12 which was the latest stable 3.x version with many important fixes.
Check the developer docs
https://ardupilot.org/dev/docs/where-to-get-the-code.html
If you just want the pre-compiled firmware
https://firmware.ardupilot.org/Copter/
I need to perform some fly mission from my own，so I have to modify the Copter codes and test them.I understand that all actions of the Copter from ifirmware itself.
Do you need to change some behaviour? or just fly a mission?
Let us know your goal if you can, people will be interested to hear about it, some may have already done it before.
Just fly a mission,like use a RC channel to enable or control that copter will fly for a while and land…
No need to worry about the code source then, just install the desired version using MissionPlanner or QGC.
3.6.12 is you best bet if you really need version 3, other wise use latest stable (4.0.7 at the moment)
and there are many fly modes such stablize,loiter,land.I consider that these modes seems like different actions.and I want integrate them together like takeoff—hover for a whlie—land.
OK, your key flight modes for RC control will be Stabilize (manual control to a degree, emergency use), AltHold and Loiter.
What you want is just to define a mission with waypoints, delays and actions and execute it.
A mission flies based on the copters current tuning and the WPNAV parameters, you dont have to specify other flight modes.
Use the latest stable Arducopter.
Definitely get practiced with the flight modes for RC control, you will need them if something goes wrong in the mission.
Read all the documentation and get everything working perfectly, then start setting up missions:
https://ardupilot.org/copter/docs/common-mission-planning.html
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As you mentioned,I really want to use latest stable Arducopter,but unfortunatly,my flow sensor only support earliest version of Arducopter accorded with sensor vender. 
That’s unlikely, what is it?
I mean it would be very rare for devs to take out support for some sensor - more get added all the time
I updated the newest fw,the flow sensor do not work，however，the earlier version is okey.
What flow sensor is it? someone might know more about it…
I dont know about the flow sensor, but all ultra-sonic range finders have been ineffective on multirotors.
There’s too much prop wash and disturbance.
You’re right,but ultra-sonic RF do effect on short range measurment,and flight controller will have a choice between ultra-sonic RF and barometer accord with different height.A low height fly under about 2 meter is ok.
You are talking about a buggy whip sensor. I don’t see how the title of this thread is relevant.
Sonsor bug is a part problem of this post,may be the title is not appropriate.
My understanding is those sonars are basically unusable for multirotors, despite what the original engineers intended. Recommend ditching the sonar and getting a lidar. I use the TF Mini Plus which is about $45. You can get some models a little cheaper, all the way up to a lot more expensive.