Abstract: A 440 nm solid-state blue laser produced by a periodically poled lithium niobate (PPLN) crystal based on cascaded third-harmonic generation (THG) is introduced. According to Sellmeier equation and phase-matching conditions of second-harmonic generation (SHG) and sum-frequency generation (SFG), two different poling periods of PPLN were designed to realize SHG and SFG at the same working temperature, and then the cascaded THG can be realized through SHG and SFG. The fundamental laser was produced by Nd: YAG, and had a 110 ns pulse width at 400 Hz pulse repetition rate with a measured beam quality of M_x²=1.81 and M_y²=2.65. It was coupled in PPLN and the 660 nm and 440 nm laser were detected after it passed through the crystal. The maximum output power of 440 nm laser was 2.4 mW where the working temperature was 55.5 ℃, and the power of 1319 nm laser was 530 mW. It is proved that a cascaded THG 440 nm blue laser can be realized by this method and it can realize a high power and high efficiency blue laser after optimization.