Abstract: For single span beam or interior span beam of prestressed concrete frame,a simplified method for calculating secondary moment was developed. A parabola was used instead of real prestressing steel bar shape in this method. The method is easy and the result is accurate. But for side span beam,the method is not accurate enough because the arrangement of prestressing steel bar in side span is different from that in interior span. It is suggested that secondary moment of side span beam should be calculated with its real shape.