Abstract: A stepped concrete beam was casted to simulate the web of a prestressed box beam with changing height, in which 10 vertical prestressing bars with different length and YGM anchors were installed, so that the test and analysis about various kinds of loss of vertical prestress were done. Pad boards with different inclined angles were plugged under the anchor nuts to analog the fixing error of the anchors, to test the relationship between the fixing angle error and prestress loss of instant anchoring. Besides, the relationship between fixing inclined angle or torque and prestress loss of instant anchoring is analyzed through changing the magnitude of the torque acting on the nut. The outcome show that the testing values of all kind of the loss of vertical prestress except for the long-term loss are greater than that calculated by the formulas in Code JTGD62-2004. The friction loss and the loss due to elastic compression of concrete member take a very small proportion of the total loss. The main loss of vertical prestress is composed by that due to anchorage slip and long-term factors, and the loss due to anchorage slip takes the biggest part. This loss is evidently affected by the fixing error of anchors and the tightening level of the screws. Higher tightening torque cannot evade the loss due to the fixing error of anchors, so the fixing accuracy of anchorage board must be improved.