A diode-pumped 424 nm blue laser is designed based on intra-cavity sum-frequency mixing. A type-I phase-matched LBO crystal is used to combine a 1030 nm Yb∶YAG laser and a 720 nm Pr∶YLF laser via sum-frequency mixing with a complex V-shaped cavity. An 18 mW, 424 nm laser is obtained by setting the pump powers of the Yb∶YAG and Pr∶YLF crystals to 5 W and 3.1 W, respectively. In this way, an all-solid-state 424 nm blue laser with a stable output is realized with an optical-to-optical conversion efficiency of 0.2%. The beam quality factors are approximately 1.62 and 1.50 in the horizontal and vertical directions, respectively.