The third-order nonlinear refractive index n2 of nitrobenzene liquid was studied using Z-scan technique under femtosecond and picosecond laser pulses. The Z-scan results demonstrate that the n2 value under 20 ps, 532 nm laser pulse excitation is 11 times larger than that under 300 fs, 800 nm excitation. The mechanism of nonlinear refraction dynamics of nitrobenzene was investigated using pump-probe with phase object technique under femtosecond pulses excitation. The nonlinear response function of nitrobenzene under ultrafast laser excitation was determined from the pump-probe experimental results. The target of the present work is to provide a reference for future work of integrated photonic device and nonlinear optical measurement based on solvent nitrobenzene.