The broadening effect of ultrashort laser pulse due to optical components is theoretically calculated and a Z-scan system with small internal dispersion based on Al-coated mirrors is reported.Some key parameters in this system are also discussed.The experiments on measuring high-order optical nonlinearities of materials are presented,the results give good Z-scan curve,and are in good agreement with reported value.Finally,a new luminous material of large third-order nonlinearity is studied with this improved system.