Both calculation and experimental results show that second harmonics of all spectral lines of Ar+ laser light can be produced simply by angular tuning ofβ-BaB2O4 crystal at room temperatures. This is used in the autocorrelation measurement of mode-locked Ar+ laser pulses, and the Ar+ laser pluse, with 4 stronger sp’ectral lines were measured to be180-230 ps at room temperature, overcoming the shortcoming of the previous SHG method at low temperatures and cross-correlation method.