A three-mode non-mode selective photonic lantern (PL) with mode conversion and lossless transmission characteristic is used to achieve coherent beam combining of a diode laser at 976 nm. Compared with the conventional spatial aperture coherent beam combining of diode lasers, the proposed beam combining field does not produce side flaps and has high beam quality. By simulating the beam combining characteristics of the PL and building a complete beam combining experimental system, final 976 nm diode laser fundamental mode output power reached 99.7 mW with a conversion efficiency of 33.2%. The experimental results show that this beam combining system achieves mode conversion and enables the diode laser to output in fundamental mode, demonstrating a promising method for coherent beam combining of diode lasers.