Cross phase modulation based time lens and four-wave mixing based time lens are realized by utilizing cross phase modulation effect and four-wave mixing effect in the high nonlinear fiber, respectively. The nonlinear process in the high nonlinear fiber of cross phase modulation based time lens is simulated and analyzed. The simulation and the analysis show that the main influence factors are dispersion, self phase modulation and four-wave mixing, which can be eliminated by using the high nonlinear fiber with a certain amount of dispersion slope. Besides, the zero-dispersion wavelength of the high nonlinear fiber should be around the centre between the signal pulse and pump pulse. Then, the nonlinear process in the high nonlinear fiber of four-wave mixing based time lens is simulated and analyzed. The simulation and the analysis show that the main influence factors are dispersion, self phase modulation and other four-wave mixing, which can be eliminated by using the signal pulse and pump pulse with a certain amount of power. Besides, the powers of the signal pulse and the pump pulse can be improved by using the high non-linear fiber with a certain amount of dispersion, then the power of the output pulse can be improved, too. Finally, two kinds of time lenes are compared with each other.