The effect of alloying element silicon (Si) on yttrium (Y) solid solution in steel has been studied by anhydrous electrolysis experiment and first-principles calculation. The effect of Y and Si solid solution on the mechanical properties of steel were studied by tensile and hardness test. The influence mechanism was analyzed by geometry and electronic structures. The results show that Si can promote Y solid solution in steel. Since the Fe–Y bond energy was less than the Fe–Fe bond, the elastic properties of Fe were reduced by adding Y. The extremely high density of electron clouds around Si improves some of the elastic properties of Fe–Y–Si. Compared with Fe, the Young ‘s modulus of Fe–Y–Si sample was decreased by 4.3 GPa and the hardness of Fe–Y sample was increased by 4.22 HV. Y reduced the interaction of the slip surfaces and Si enhanced it, thus changing the plasticity of the steels.