The position adjustment of power transformer tap changer is an important means of voltage regulation in the power system. When the tap changer is not in the standard gear, the transformer works under non-standard ratio conditions, and its operating electrical parameters deviate from the rated parameters. The existing non-standard ratio transformer model is difficult to guarantee high accuracy and high calculation efficiency at the same time. Aiming at this problem, an improved Π-1 type non-standard ratio transformer equivalent circuit model is proposed by using the equivalent transformation of the circuit network. The proposed model has the same accuracy as the accurate T-type non-standard ratio transformer model. And it avoids the disadvantage of increasing the number of equivalent nodes which guarantee high computational efficiency. In order to further improve the convergence characteristics of power flow calculation for backward/forward sweep method, an improved Π-2 model with better practicability is derived based on Y-△ impedance network transformation, which increases the practicability of the model. Based on the comparative analysis of simplified 2-bus, modified IEEE 9 and IEEE 300 test cases, the validity and superiority of the proposed model in terms of calculation accuracy, speed and convergence performance are verified.