The impulse turbine, which is a part of an oscillating wave column, plays an indispensable role in wave energy conversion. In this paper, a self-rectifying impulse turbine has been investigated numerically under reciprocating flow and the quasi-steady analysis method of the turbine has been studied. Firstly, the numerical model was validated by corresponding experiment results. Then, the reciprocating flow condition was accurately simulated with the method of setting double velocity inlets. The results showed that there existed hysteretic loops in turbine performance curves. And different pressure drops in acceleration and deceleration led to this hysteretic characteristic. Finally, in order to investigate quasi-steady analysis method, the frequency and amplitude of the reciprocating flow were changed respectively and the results were compared with those of steady flow. It has been found that hysteretic loops are distributed around the curves of steady flow. When the frequency is lower or amplitude is smaller, the results of reciprocating flow are closer to those of steady flow. In conclusion, the performance of steady flow can be used to predict the performance of reciprocating flow with little velocity change in a short time, which reveals the feasibility of quasi-steady analysis method.