The residual stress was measured by X-ray diffraction method (XRD) for large-size and middle-thick TA15 Ti alloy plate. By scanning the (213) plane of TA15 titanium alloy by X-ray diffraction, we observed the changes in peak shape, and usedthesin₂ymethod to calculate the residual stress. Residual stresses at some different locations on the plate surface have the larger deviation under the normal diffraction conditions,so that the test results are not reliable. On thebasis of the range 30 °～85 ° diffraction scans on samples,it is regarded that big deviation of stress is resulted from the coarse grain. So we propose that increasing theX-ray irradiating area is an appropriate way to decrease the deviation of the residual stress.After improvement, the results show that the surface of TA15 titanium alloy plate exists compressive stress after hot rolling. The surface residual stress range is from -250 MPa to -450Mpa,and measuring deviations were within ± 50MPa, which is highly reliable. The further analysis of the formation mechanism is conducted on TA15 plate residual stress.A theoretical basis and experimental data are supplied to eliminate the residual stress.