This paper introduced a realisable method of multiwavelength pyrorneter. The unknown and variety of surface emissivity of an object is the main cause which affects the accuracy of the radiation pyrometer indication. The multiwavelenth pyrometer is one of the effective methods to decrease the effect of the emissivity and acquire the true temperature of the object surface. On the basis of synthesizing many references, the author discussed the theory of multi - wavelength pyrometer in detail, analyzed some algorithms of data processing, and designed and reallsed the fourcolour pyrometer.