In this paper we report a general system for measuring wavelengths of lagers, both CW lasers and pulse lasers. The system consists of a grating spectrometer and a one-dimension optical multichannel analyser. We use a low-density grating and operate in high-order diffraction. The distance between the He-Ne laser line and the laser line to be measured on the output spectral plane is read out in real time by the array of the OMA, the laser wavelength is then calculated by a personal computer with an error of 0.001 nm An analysis on the error is presented.