Abstract: The grid connected photovoltaic（PV） power is booming in northwest of China, and large-scale PV power is mostly integrated to grid through long transmission lines. However, PV systems may face the threat of sub synchronous oscillation（SSO） when AC system strength is weak. This paper established the sequence impedance model of grid connected PV system; mechanism and characteristic of SSO in PV plants integrated to weak AC networks were delved into through impedance based analysis method. The results show that, under certain conditions, the impedance of PV system is capacitive in the sub synchronous frequency domain, when the networks strength is weak while the PV capacity is high, the resonance may occur between the capacitive PV system and AC grid. In addition, a smaller proportional gain of current loop may increase the risk of SSO, however, a bigger proportional gain and integral gain of phase locked loop can ease the SSO. Finally, time domain simulation based on PSCAD/EMTDC was conducted to validate the results of the impedance based analysis. The research can provide guidance to the project of scaled PV plants integrated to weak AC networks in some sense.