In order to obtain the tunable dual-frequency laser with large frequency difference, a laser diode endpumped dual-frequency Nd∶YAG laser with tunable frequency difference has been designed, using the light splitting and polarizing functions of the polarizing beam splitter (PBS) to form perpendicular straight and right angle standingwave cavities. Both cavities employ birefringent filters acting as laser axial mode selectors. The p-and s-component of 1064 nm laser light can therefore be forced to oscillate simultaneously in single longitudinal mode. The principles of single longitudinal mode selection by birefringent filter and the simultaneous oscillation of the dual-frequency laser have been theoretically analyzed, and the characteristics of Nd∶YAG laser with single longitudinal mode oscillation at 1064 nm and frequency difference tuning have been investigated experimentally. It has been observed experimentally that the frequency difference of the dual-frequency laser at 1064 nm is tunable in a range from 11.4 GHz to 168.6 GHz, the maximum frequency difference has reached the fluorescence line width of Nd∶YAG. Such a dual-frequency Nd∶YAG laser can be used for laser interferometric measurement, laser spectroscopy and other fields.