Soft phase-locked loop (SPLL) gets more and more applications in the field of modern industrial control, especially in grid voltage phase locking. This paper analyzes the basic structure and working principle of the SPLL and designs an improved grid SPLL according to the shortcomings of traditional SPLL which has large amount of computations and is difficult to achieve on the chip of digital signal processor (DSP). On one hand, the control algorithm is simplified based on the locked nature of the phase angle by reverse thinking. And these builds on the very understanding of the relationship of three-phase voltage coordinate transformation. On the other hand, the PI parameters are calculated dynamically (not the constant PI parameters) which makes phase locking function work dynamically and the control algorithm is more simple. The theoretical analysis and simulation analysis show that the modified SPLL is simple, possesses better performance in dynamic phase tracking, harmonic suppression and detection of unbalanced voltage, etc. , can deal with fault such as power down, and is very appropriate to implement on the DSP.