Distributed Raman Fiber Amplifier (DRFA) is widely used in modern communication systems because it has a series of excellent features. Considering the existing problems of DRFA gain control, this paper studies and improves a method of Automatic Gain Control (AGC) of DRFA by Amplifying Spontaneous Emission (ASE) out of band. In view of the fact that DRFA gain is greatly affected by the performance of optical fiber link, this paper firstly theoretically analyzes the influence of the joint loss at the position 0 away from the pump source in the optical fiber link on the Raman gain control. Then the splice loss at different positions from the pump source is set to be equivalent to the splice loss at position 0. The relationship between Raman gain and out-of-band ASE power is modified so that the AGC of DRFA can be realized more accurately. The Optical Time Domain Reflectometer (OTDR) function is integrated in the DRFA module to detect the distance between the joint loss and the pump source and the loss value in the engineering optical fiber link. Experimental results show that the proposed AGC method can control the influence of joint loss on DRFA gain within 0.2 dB.