I writing a python script to adjust flight path angle to reach certain waypoint in AUTO mode. As I understand TECS_PITCH_MIN and MAX are related to pitch attitude angle.
how can I control the flight path angle instead ?
Ardupilot alredy uses glideslopes for altitude changes above GLIDE_SLOPE_MIN (default 15m) for small changes it uses an immediate altitude changes, so if you set GLIDE_SLOPE_MIN to 1m it will pretty much always do a glideslope change, setting to 0 disables glide slope.
My task is to guide a plane from certain altitude (WP1) towards a ground net WP2 (placed on the ground, altitude = 1 m). As the plane approaches WP2 at around 10:20 m above, it sort of glides upward and switch mode to RTL and never reach the net.
That is the expected behavior if the mission ends in a non landing waypoint, you need at least a run out waypoint to delay RTL past touch down.
Though I strongly recommend using landing waypoint as it arms Ardupilot for landing including enabling semiautomatic go around and post landing disarm. Configure landing for tight altitude control and use landing waypoint altitude to hit the net while flaring.
@LupusTheCanine ,thanks for the suggestion.
I feel struggled with understanding the landing config. parameters. Is there a more detailed description for these parameters?
I tried to configure some parameters, but the plane descends too smoothly that it always passes the net with a big altitude gap. I want to maintain a fixed glide slope (or more precisely, fixed flight path angle). I think no flare is needed.
Well, landings are supposed to be smooth 
.
“Flare” is needed to kill the motor prior to touchdown (prop, motor and ESC will appreciate the motor not stalling in the net), you can configure automated landing to maintain descent rate in flare, the default sink rate is 0.25m/s which appears to be too low for hitting the net effectively.
I even set that abruptly to 90 m/s and the apparent change is minor. 
I’m sure the plane can experience around 25 m/s sink rate, but I cannot get that achieved in the landing phase.
There also is a pitch limit. Do you have a log that shows it?