The theoretical model of a carrier-injection-PIN-based submicrometer-scale silicon waveguide optical phase modulator is built. The optical and electrical properties of the modulator are analyzed theoretically, and the conditions of single-polarization and single-mode for the submicrometer-scale waveguide are determined based on the theoretical model. The effects of structure dimensions and doping conditions on modulation efficiency are discussed emphatically under the single-polarization and single-mode conditions. The analysis shows that modulation efficiency can be improved effectively by reducing slab height, increasing doping concentration, increasing doping depth and reducing distance between the doped regions and the rib edge. The optimized device scheme is presented based on the analysis results. The excellent optical mode overlap with the refractive index change region, together with the submicrometer-scale waveguide, enables the modulation efficiency of 19 rad·V-1·mm-1, the modulation bandwidth beyond 1 GHz, and the advantages of compactness, low voltage and ease of integration.