In order to study the effect of molecular weight of chitosan on the food preservation performance of in-situ modified nano CaCO3-chitosan composite coatings， the composite coatings of chitosan with different molecular weight were prepared by tape casting， the characterization of SEM， TEM， XRD， and FT-IR were analysed， the physicochemical properties were determinated， and the preservation performance of composite coatings were determinated using Sciaenops ocellatus as freshness object. The results showed that the phenomenon of agglomeration ccould be formed in the in-situ modified nano CaCO3 particles when the molecular weight of chitosans was 60 kU. The molecular weight of chitosan was 100 kU， which results in uneven dispersion and agglomeration in in-situ modified nano CaCO3 particles in the coating， moreover， the section of coating was not smooth and there were cracks and pores. When the molecular weight of chitosan was 160 kU， the size of the nanoparticles in composite coating was even and suitable， evenly dispersed， basically no agglomerative， and good compatible with chitosan， and which physical and chemical properties were better. Compared Sciaenops ocellatus which were untreated and the chitosan molecular weight of 60 kU composite coating， the freshness index and structure index of Sciaenops ocellatus which were treated with the chitosan molecular weight of 160 kU composite coating were slowly down. That indicated it could effectively ensure the quality of Sciaenops ocellatus and prolong the shelf life.