Experimental research was conducted on the transverse stress of 32m post-tensioned precast simply supported concrete box girder with ballastless track. Vibrating wire strain gauges were embedded in the key sections to monitor the concrete transverse strain. The concrete strain development law was analyzed. The monitored results showed that there were low transverse tensile stresses and the cracking resistance of the girder was enough during pre-tensioning stage. The research findings provided effective experimental data for design and construction quality control.