On the basis of the relationship between the point spread function (PSF) and the small angle scattering function (SASF) of the sea water, A new type of scatterometor for measuring SASF of sea water in situ was built. It can measure F PSF of the sea water body using this instrument and the calculated SASF of seawater from PSF data. The angular resolution is 0.01°, accuracy is 5% and the structure is simple and steady. The data of small angle scattering of sea water in typical area of the East China Sea was abtained.