The instability of precast prestressed concrete bridge girders during construction have been of particular concern to bridge engineers. After they are installed on bearing supports, prestressed concrete girders are immediately subjected to environmental thermal loads that may be exacerbated by fabrication and construction errors. Thus, in this research, the environmental thermal loads, which cause extremes in thermal deformations in precast prestressed concrete girders, were determined. Then a three-dimensional nonlinear finite element sequential analysis procedure was developed to evaluate the behavior of a precast prestressed concrete girder subjected to both thermal loads and geometry and support imperfections during each construction stage. This analysis indicated instability in a 30-m long prestressed concrete BT-1600 girder when total lateral deformation in the middle height of the girder at mid-span exceeded about 25cm.