The attacker also tried to snatch the cop's weapon after injuring him in the neck. He was overpowered and taken into custody, they said.
The injured policeman is undergoing treatment at Soura Medical Institute here and his condition is stated to be "critical".
A police party was on duty in Pampore town, 16 kms from here, on Srinagar-Jammu national highway when the suspected militant attacked one of them and tried to kill him by slitting open his throat.
They said the attacker was taken for questioning and his identity was not known yet.
The incident, first of its kind in the valley, sparked panic in the area. The region has been cordoned off and further details are awaited, police said.