The conventional Receiver Autonomous Integrity Monitoring (RAIM) algorithm is limited when detecting weak pseudo-range bias under gradual change because of its longer detection delay and higher miss detection rate. A weight modification accumulated epochs parity vector RAIM algorithm based on self-adaptive strategy was presented to solve this problem. In this algorithm, the weight factor was obtained according to the single epoch fault degree to adjust the proportion of each epoch in the selected window to structure more effective detection statistics, and the size of the window was determined according to the repeated simulation experiments. The simulation results show that the proposed method can better detect weak pseudo-range bias under gradual change, compared to accumulated epoch and the conventional RAIM algorithm, the detection delay time declines by 16.67% and 56.52% respectively.