Oscillations in current control (CC) timescale (higher than 50 Hz) caused by voltage source converters (VSCs) integrating into the weak grid have occurred. However, relevant studies lack enough mechanism cognitions of instability of such system. In order to guide the parameter setting of the controllers, this paper first presents a small signal model of VSC based on motion equation concept. Secondly, a SISO equivalent method which can quantitatively analyze the interaction between variables in MIMO is introduced. Thirdly, the influence of phase locked loop (PLL) bandwidth, current controller parameters in the d-axis and q-axis and filter bandwidth on the feedforward branch of terminal voltage on stability is analyzed based the SISO equivalent method, which can help engineers to study the mechanisms of the voltage amplitude/frequency stability. Finally, the simulations in time domain verify the correctness of the analyses.