Recently, organometal trihalide perovskites (OTPs) are very prospective photovoltaic materials because of its low-cost, easy fabrication and high power conversion efficiency (PCE). However, the low stability of perovskite solar cell is one of the important problems to be solved for their commercialization. Among all those factors that affect the stability of OTPs, ion migration can be important. The presence of ion migration has received widespread attention since the report of photocurrent hysteresis of OTPs. According to experimental data, the ion migration must be an important factor for many unusual phenomena in OTP materials and device, such as current-voltage hysteresis. The direction of the diode polarity and photocurrents can be reversed repeatedly by applying a small electric field. Here we show that how ion migrates and what the effect of ion migration is.