The transient thermally induced nonlinearity of a novel metal cluster polymer solution was investigated by using the phase object （PO） Z-scan technique which can distinguish the transient thermal effect from the third-order nonlinear refraction easily in the nanosecond regime. We studied the optical nonlinearity of the ［Tp*W（μ3-S）3Cu3Py3（μ3-Br）］（PF6）/DMF solution using both PO Z-scan and conventional Z-scan techniques with the laser pulse width of 8 ns under different input pulses energe. Results of numerical simulations and solution of simultaneously acoustic wave equation and the diffusion equation of conduction of heat agree well with the experimental results，which shows that the nonlinear refraction of the sample is due to transient thermally induced nonlinear refraction.