The principle of producing dual-wavelength laser was presented in laser crystals of four energy level system with double lower level. The threshold formulas of dual-wavelength simultaneous oscillation in two-mirror cavity were introduced. It was found by calculation that the existence of space hole-burning effect made the threshold condition widened to a region from a curve, which was helpful for dual-wavelength oscillation. Moreover, the effect of the crystal length and pump power on the dual-wavelength threshold were calculated, results show that, before the gain saturation happens, the longer the crystal length is and the higher the pump power is, the more easily the dual-wavelength laser will appear. Finally, the dual-wavelength threshold curves of several common Nd-doped laser crystals were calculated, which showed by comparison that Nd:YAP crystal is more suitable for producing dual-wavelength laser due to its two similar emission cross sections.