By means of Fourier transform infrared spectrometer(FTIR), super deep scene microscope, scanning electron microscope(SEM), energy dispersive spectrometer(EDS), X ray diffraction(XRD) and thermal gravimetric analyzer(TG DTG), the characteristics of biofilm formed on mortar surface in seawater environment and its influence on mortar performance were studied. The results showed that sulfur oxidizing bacteria in seawater would adhere to the mortar surface and form a large number of biofilms, which were composed of polysaccharides, proteins and microorganisms. After 15d, the biofilm began to reproduce rapidly. After 75d, the biofilm gradually fell off, and after 90d, the new biofilm reattached. In the process of reproduction and evolution, the biofilm would consume a large amount of oxygen in seawater. By studying the effect of the presence or absence of biofilm on mortar performance, it was found that at 120d, the compressive strength of the biofilm free samples was 525MPa, with a strength growth rate of 038%, and the mineralized products contained a large amount of gypsum. However, the compressive strength of the biofilm free samples was 681MPa, with a strength growth rate of 3172%, and the mineralized products contained a small amount of gypsum. This indicates that the presence of biofilm affects the transmission of corrosive medium in seawater and has a moderating effect on the corrosion of mortar.