In order to solve the problem that it is difficulty to accurate calculate the three-phase current unbalance factor when missing the phase information, a method based on the effective value of fundamental current was proposed. Firstly, this paper analyzes the calculation method of three-phase current unbalance factor based on symmetrical component method, and the difference of four commonly used engineering calculation methods of three-phase current unbalance factor when missing the phase information; Secondly, based on the decomposition law of symmetrical component method, the three-phase unbalanced current was decomposed into two groups of current phasors which is phase symmetrical, the calculation formula of current unbalance factor based on the effective value of fundamental current is derived when the phase symmetry, then determine the master-slave relationship between two groups of current unbalance factor under different working conditions; Finally, the applicability and difference of the calculation method in the calculation of three-phase current unbalance factor are analyzed based on the unbalanced characteristics of load current in the low-voltage district, and the operation data in the low-voltage district are measured and verified, Compared with the four commonly used engineering calculation methods, the calculation results of the method and the symmetrical component method are more close and basically consistent.