NOVELTY - A polymer composition comprises a propylene polymer and a film forming auxiliary agent. The polymer contains 70-96 wt.% structural unit derived from propylene, 1-10 wt.% structural unit derived from ethylene and 1-20 wt.% structural unit derived from 4-8C alpha -olefin. The polymer is obtained by gas phase olefin polymerization of propylene, ethylene and 4-8C alpha -olefin in a gas phase reactor under contacting using an olefin polymerization catalyst. The olefin polymerization catalyst comprises a solid component, an external electron donor compound and an organic aluminum compound.    USE - Polymer composition is used for solar cell sealing film (claimed) and heat-sealing layer for heat-sealing membrane or multilayer film.    ADVANTAGE - The polymer composition provides polymer film having low initial heat-sealing temperature of less than 115 degrees C.    DETAILED DESCRIPTION - A polymer composition comprises a propylene polymer and a film forming auxiliary agent. The propylene polymer contains 70-96 wt.% structural unit derived from propylene, 1-10 wt.% structural unit derived from ethylene and 1-20 wt.% structural unit derived from 4-8C alpha -olefin. The propylene polymer is obtained by gas phase olefin polymerization of propylene, ethylene and 4-8C alpha -olefin in a gas phase reactor under contacting using an olefin polymerization catalyst. The olefin polymerization catalyst comprises a solid component, an external electron donor compound and an organic aluminum compound. The solid component comprises a carrier, a titanium compound on the carrier and an internal electron donor compound. The particle size of the solid component is 8-150 mu m. The particle size distribution curve of the solid component has single peak distribution. The propylene polymer has xylene soluble content of 2-20 wt.% and xylene insoluble content of 80-98 wt.%. The xylene soluble substance has crystallization temperature of 25-65 degrees C and melting point of 80-100 degrees C. The xylene insoluble substance has crystallization temperature of 85-105 degrees C and melting temperature of 125-135 degrees C.