The temperature -stress characteristics of passive devices (ion exchange planar optical waveguides) is investigated employing the self imaging theory and numerical simulation methods with the 1×5 PLC-MMI optical power splitterconsidered as an example The dependences of the insertion loss and additional loss of each port of the device on the length and refractive index change of multimode waveguide region when the device operates at 850 nm are studied. The waveguide deformation and index change due to the environmental temperature and stress are analyzed, and consequently, the splitting ratio of the MMI splitter is discussed due to the optical fiber-waveguide coupling loss and birefringence. All the results indicate that the developed 1×5 PLC-MMI optical power splitter demonstrates stable performance with excellent immunization to the influence of environment and stress.