The constable, identified as Ravi Dutt (30), was checking documents of heavy moving vehicles at the picket last night when the incident took place, said a senior police official.
Dutt was rushed to the AIIMS Trauma Centre and his condition is now reported to be stable.
The truck was approaching from Mahipalpur area and heading towards Dhaula Kuan.
It rammed into the picket, after which the driver abandoned the vehicle and fled the scene.
The police have impounded the truck and the owner of the vehicle was questioned today.