Transient spin grating, as a novel technique in ultrafast magneto-optical laser spectroscopy, is widely used to study the properties of spin transport in semiconductor low-dimensional structures. The principle of transient spin grating and the optical arrangement are described briefly in this paper, which includes the generation and detection of transient spin grating. The signal to noise ratio was significantly enhanced by using heterodyne detection. The spin diffusion coefficient Ds was determined by this technique to be about 551cm/s in (110) GaAs/AlGaAs single quantum well at room temperature.