Nd-doped phosphate glass twin-core fiber (TCF) is prepared by stack-and-draw method. By combining with rod-in-tube technical, a novel design to produce TCF with a various ratio of core diameter to their separation is obtained. The TCF laser employs an 808 nm laser diode (LD) as pump source, a phosphate TCF with a length of 6 cm and a diameter of 620 μm is gain medium, and a resonant cavity is formed by a broadband high reflector and Fresnel reflection provided by perpendicular cleaved end of the active TCF. Above laser threshold, clear and stable interference strips are observed by CCD in far field, showing that TCF laser operates in self-phase-locking. A maximal output power of 52 mW and a slope efficiency of 27.1% with respect to absorbed pump power are obtained. Characteristics of the TCF laser spectra at different output power are also investigated.