Previous particle-in-cell simulations have evidenced that quasi-perpendicular shocks are non-stationary and suffer a self-reformation on gyro scale of the incoming ions. In this paper, by separating the incoming ions into reflected and directly transmitted parts, we investigate ion acceleration in a non-stationary perpendicular shock. The results show that shock drift acceleration (SDA) is a dominant acceleration mechanism, while shock surfing acceleration (SSA) mechanism becomes more and more important with the increase of the initial particle energy (both their average final energy and percentage increase). The percentage of reflected ions cyclically varies in time with a period equal to the self reformation cycle.