Amrit Bora (28), who was posted at the Kamalabari UBI branch, tried to shoot himself with his service rifle while on guard duty last night, they said.
Some of his colleagues happened to witness the attempt and tried to snatch the gun.
However, a bullet was released from the gun during the scuffle and hit Bora in his left arm.
Bora was admitted to Jorhat Medical College, where his condition was stated to be out of danger.
The reason behind the constable's suicide attempt was yet to be ascertained, police said.