The invention disclosesa prefabricated PCF external wall board with a window frame and an installing method thereof. The wall board comprises a superposed beamprefabricated portion, the window frame, a window lower wall and a PCF external sash board extending towards the periphery of the window frame, wherein window side edge boards are arranged on the left and right side edges of the window frame, the superposed beam prefabricated portion is located at the upper portion of the window frame, and the PCF external sash board upward extending of the window frame is located on the outer side of the superposed beam prefabricated portion. The window lower wall is located at the lower portion of the window frame, the inner side of the window lower wall is an inner sash board, and the PCF external sash board downward extending of the window frame is located on the outer side of the inner sash board of the window lower wall. A grouted layer is arranged below the window lower wall, the window lower wall is provided with a grouting hole and a grout outlet hole, and the grouting hole and the grout outlet hole are communicated with the grouted layer. A site construction process can be simplified, the site construction time can be shortened, and the advantages of prefabricated parts can be fully played. In addition, connection between the window frame and peripheral members is reliable, the waterproof performance of the window frame is superior, and the water seepagehidden danger of a window can be completely eradicated.