The invention discloses a voltage-crossing fault current calculation method for crossing from single-phase to double-phase by mixed-voltage one tower four-parallel lines, comprising steps of respectively calculating a positive sequence equivalent impedance and a negative sequence equivalent impedance of two systems having different voltage levels to obtain a positive sequence network and a negative sequence network by a strong current low intensity magnetic system, performing unified decoupling on zero sequence networks of two systems, performing unified reduction to obtain an unified zero sequence network, and performing a faulted current calculation when the voltage-crossing fault happens during the process of crossing from the single-phase to the double phase, dividing the strong current low intensity magnetic system into a low intensity current strong magnetic part and a current source formed by a transformer, and superposing the low intensity current strong magnetic part and the current source to obtain the faulted current when the electrically-connected strong current low intensity magnetic system is faulted in voltage crossing. The invention is applicable to the single-phase crossing to double-phase voltage crossing calculation, can effectively solve the complicated coupling between two systems when the voltage crossing is faulted and the large calculation amount of the super-high-order decoupling method, and can provide a solid theory foundation to the relay protection for the mixed-voltage one tower four parallel line voltage crossing fault.