A low insertion loss and multiband B-sandwich radome wall is designed specially for mobile satellite communication system which works at Ka-band，K-band and Ku-band, by using the four-port transmission formula of microwave network. Transmission coefficient,reflection coefficient and insertion phase delay are analyzed. When the angle of incidence is 0—40°, transmission coefficient is better than 95% within Ka-band, better than 88.8% within K-band and Ku-band, reflection coefficient is less than 6.3% at all band，and the difference of insertion phase delay between vertical and horizontal polarization is very small.