Mo5SiB2 is an ideal candidate for high temperature material. Although the D8l‐Mo5SiB2 has been published, the other phases and the related properties of Mo5SiB2 are still unclear. Here, two possible Mo5SiB2 phases (D8m‐Mo5SiB2 and Cmcm‐Mo5SiB2) are predicted by using the first principles method. The structural stability, mechanical properties, melting point and electronic structure of Mo5SiB2 are studied. The results show that the D8m‐Mo5SiB2 and Cmcm‐Mo5SiB2 are thermodynamically and dynamically stables. The calculated bulk elastic modulus, shear modulus and Young's modulus of the Cmcm‐Mo5SiB2 are close to D8l‐Mo5SiB2. In addition, the predicted Cmcm‐Mo5SiB2 has high melting point (2518.5°C) compared to the D8l‐Mo5SiB2 and D8m‐Mo5SiB2. In particular, the predicted D8m‐Mo5SiB2 exhibits better plasticity than the D8l‐Mo5SiB2. The calculated density of states and Mulliken overlap population shows that the D8m‐Mo5SiB2 and Cmcm‐Mo5SiB2 all exhibit metallic behavior. The metallic behavior mainly depends on the electronic interaction between Mo atom and B atom near Fermi level. Furthermore, the bond length of Mo‐B bond in the D8m‐Mo5SiB2 is longer than the other two structures, which may be the reason why the ductility of the former is better than the latter. This work investigates the structural, mechanical and melting point of three Mo5SiB2 by using the first‐principles calculations. The results show that the two novel Mo5SiB2 (D8m‐Mo5SiB2 and Cmcm‐Mo5SiB2) are predicted. Three Mo5SiB2 exhibit high bulk modulus and strong elastic stiffness. In particular, these ternary silicides also show better ductility and high melting point. The calculated melting of Cmcm Mo5SiB2 is up to 2518.5°C, which is higher than the other Mo5SiB2.