Currently available oscillation modeling methods are all based on changing the“average frequency of 0- t interval”，which cannot directly reflect the actual state of a power system. To solve the problem， this paper proposes a new modelingmethod based on instantaneous frequency. By fitting the changes of instantaneous frequency under constraint conditions， themethod can accurately simulate the oscillation process of a real system without introducing the“average frequency of 0- tinterval”. With this method， an oscillation model reflecting the whole process is developed， with two typical cases of oscillation （without/with fault） simulated using the power system block（PSB） in MATLAB software. The simulation results show thatthis model can satisfactorily match the preset curve of frequency during oscillation， and is ideally suitable for research on theperformance evaluation of relay protection.