Development of wide area measurement based online closed-loop control is an important step to improve the security and economy of power system operation. Focused on the mid-term voltage stability， an online coordinated voltage control scheme is proposed. An estimated model is firstly established based on the quasi-steady-state model to grasp the main dynamics during the process， a direct multiple shooting based simultaneous direct dynamic programming algorithm is then employed to solve the nonlinear optimization problem. Due to the nonlinear characteristics of the model， a large amount of control components， and the requirements of the online application， the techniques of initial value embedding and real-time iteration are further introduced， so that the nonlinear characteristic is preserved， and calculation burden and time consuming is remarkable reduced. Different control candidates including on-load tap changer（OLTC）， automatic voltage regulator（AVR） settings and load shedding schemes are well coordinated to enhance the security of the system under critical conditions. The validation and effectiveness of the proposed scheme is verified by simulation analysis on the New England 10-generator 39-bus testing system.