An athermal design for Solc-type filter based on periodically poled KTP (PPKTP) is proposed. The athermal static phase retardation (ASPR) direction in KTP along which the quasi-phase-matched (QPM) condition is insensitive to the temperature is found. The optimal design for Solc-type filter along ASPR direction is obtained. The coupling theory of QPM linear electro-optic effect is employed to study the polarization coupling in PPKTP. The study results demonstrate that the central passing wavelength of Solc-type filter keep unchanged with an almost 100% output when the temperature varies from 268 to 328 K.