Based on phasor measurement unit（PMU） measurements， an online algorithm is presented to calculate power transformer's winding reactances and transformation ratio. It is found that due to the random error of PMU measurements， the multi-period parameter calculation results comply with a normal distribution. Furthermore， based on the statistics theory of normal distribution， a robust parameter identification algorithm is proposed to give confidence interval and point estimation of transformer's parameters and transformation ratio. Considering that low voltage side of three-winding transformer is usually connected with reactive power compensation devices， the algorithm can precisely identify the tap changer position without low voltage side PMU measurements. Examples from a wide area measurement system（WAMS） in North China power grid show that the methods proposed is feasible and reliable.