I would like to know the reason of trigger push of..as I have this problem in my gun..I changed springs but no result.

If the action has had "work done", the single action engagement surfaces on the hammer and trigger are not working properly. This can also happen in a gun that has had no alterations made. Either way, it is a dangerous situation and a call to factory customer service is in order; since it is a safety issue they 'should' address it quickly and at no charge to you if the gun hasn't been modified.

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Alan
SWCA LM 2023, SWHF 220

I understood, but my question is just to know the reason of the trigger push of..and why happen , even if the trigger spring is more week than the original..

If the OEM spring tension is in tact, the reason for the push off is that either the bevel on the trigger, or the SA cocking notch in the foot of the hammer has been compromised.....either by wear and tear, or modification.

The single action pull weight is dependent on both the proper angle and sharpness of the trigger bevel, and the integrity of the SA notch of the hammer. OEM spring tension exerted by the rebound slide spring and the mainspring serve to hold the bevel in the notch at the proper tension until the trigger is acted upon by the user/shooter.

If the bevel or notch (or both) becomes worn, or the proper spring tension is not maintained, the bevel can slip off (out of) of the hammer notch with nominal pressure on the hammer spur.

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Ret. LE, FA Instr, S&W Armorer