The properties of the propagating field in multimode photonic crystal waveguides exhibiting no photonic band gaps with triangular lattices are investigated. By the effective refractive index method, these structures are regarded as three layer planar dielectric waveguides. And the multimode interference effect is numerically analyzed and simulated based on the self-imaging principle. The results show that the field can propagates effectively and resembles index-guided mode due to the combination of total internal reflection and distribution Bragg reflection. High transmission region shifts toward the shorter wavelength region, the bandwidth and the transmissivity increase with the increase of filling ratio, which provides a theoretical reference for selecting an appropriate filling ratio in practical applications.