At present， the measurements of the wide area measurement system（WAMS） and supervisory control and data acquisition（SCADA） are both available in power systems， but the measuring accuracy of the WAMS is much better than that of SCADA. Hence， a linear dynamic state estimation algorithm is proposed to deal with this problem. The measurements of the active and reactive power flow and power injections at the nodes of the SCADA system are transformed into the equivalent current phasors by measurement transformation， so that these measurements can be combined with those of WAMS and form a mixed measuring system. The algorithm proposed is denoted by rectangular coordinates and the linear exponential smoothing technique and linear time-invariant Kalman filtering method are used to implement the forecasting and estimation. Since the proposed algorithm has a constant Jacobian matrix， the calculating time can be significantly reduced and the calculating accuracy ensured. Simulations are carried out in an IEEE 14-bus test system to demonstrate the validity and advantages of the algorithm proposed.