do any one know the definition of stat_FLT TIME, how do flight recorder count the flight time? Does it begins to record the FLT time according to the motor start or the altitude？And please look at the attached photo, the stat_reset is 8.65, what is that mean？
Read the official documentation: http://ardupilot.org/plane/docs/common-flight-time-recorder.html
Does this provide the answers you want?
the official document did not describe much more detail I think , I need the flight time maybe I have to change the bearing of my motor ,but what is the flight time definition it didn’t tell .in the real plane ,the flight time begins to accumulate after the wheel is above the ground.
may be i just go extream to know such a accurate value 
another things is that I have set the STAT_RESET to 0，but all the stat value is still 0 after several times connect to my pc via USB cable ,is that normal?
This is the same for ArduPlane. There is a sub-system that attempts to determine when the aircraft is flying (rather than sitting on the ground, for example). If the aircraft is flying, the FLTTIME is counting.
I am not sure. From looking at the source code, the AP_Stats is updated as a part of the plane scheduler in ArduPlane.cpp. I don’t know if only connecting via USB cable is enough.
If you can, try powering-on your system outside where you have GPS, and getting it to arm. Then disarm without flying and power it down. Then, connect via the USB cable and check if the STAT_RESET parameter is still 0?