Now, higher loss in polymer modulators is one of important problems to be solved in the research of electrooptic modulators. The waveguide loss plays an important role in the insert loss. Especially, modulators will inevitably have higher waveguide loss seeking high EO coefficients. It is an important topic to improve the modulators' loss. An idea is proposed that light-wave modulated only in claddings. Analysis shows that this method can decrease largely waveguide loss, so it helps to improve the insert loss. Because the light power in claddings is weaker than one in core, the overlap will weaken when modulation is realized only in cladding. It is found that optimizing the dimensions of waveguide can largely reduce this weakening. Calculation makes the optimum overlap to 0.89. The length of interaction can increase with the decreasing of waveguide loss, which is helpful for decreasing half-wavelength voltage.