The relativistic mean field theory is used to investigate the shape evolution of Pt isotopes. The calculated binding energy and deformation parameter 2 are consistent with those obtained in experiment. The potential energy surfaces and the single particle levels show the shape evolution for Pt isotopes. From N=88 to N=126, the shapes for Pt nuclei evolve from spherical shapes to X(5), and then to shapes of stable quadruple deformation, finally back to the spherical shapes. In detail, 166-172Pt are spherical. 174Pt and 192-196Pt possess the X(5) symmetry. 176-190Pt are deformed nuclei. 204Pt holds spherical shape. These results in agreement with the experimental observations.