Various performances of the two-prism beam expanding system in dye laser resonator are studied. The system possesses not only good beam expansion and collimation for the laser beam, but also good filtering and tunning functions. The angular dispersion formula of the two-prism beam expanding is derived and proved in theory, by using a tunning resonator composed of such a system without etalon, the narrowband tunning laser output with bandwidth ~GHz (~0.001 nm) can be achieved. The laser output bandwidth <4 GHz is experimentally obtained. The theoretical basis (angtilar dispersion formula of a multiprisms system) for getting <1 GHz narrowband laser is given too. In addition, advantages of the double(mixlti) prisms system-small reflection loss, polarization operation, easy adjust are analyzed. It indicates that two(multi)prisms beam expanding system is on ideal optic component with multifunctions (expanding, filtering and tunning).