The nonlinear optical properties of P3HT in orthodichlorobenzene were investigated by Z-scan technique using second harmonic generation (532 nm) of mode-locked Nd:YAG laser in the picosecond domain. The experimental results show the magnitude of their nonlinear refraction indices was up to the order of 10-11 esu. The reverse saturable absorption of P3HT solution was observed and their nonlinear absorption coefficients reach up to 3.4 cm/GW. The strong optical nonlinearity of P3HT may find its new application in the photoclectric field.