A state estimation model is proposed for three-phase four-wire area load. Considering the three-phase node injection current, neutral virtual and voltage amplitude measurement equations of power source and load endpoints, and three-phase zero injection equality constraint equations of T endpoints, a weighted least squares theory state estimation model with equality constraints is developed, which takes the real and imaginary parts of three-phase a, b, c and neutral voltages as state variables. Newton method is used to solve the presented model, and the modified IEEE-13 bus system is used for simulation. The results show that it is effective to solve state estimation problems for area load, and the method shows reliable convergence, fast speed, thus can meet engineering requirements.