Betaine surfactants have been widely used as oil displacement agents for tertiary oil recovery due to their unique amphiphilic structure,high surface activity,low critical micelle concentration and good emulsification performance.To investigate the effect of alkyl carbon chain number on the chemical flooding performance of betaine surfactants,5 kinds of betaine solutions with alkyl carbon chain number (n) of 12,14,16,18 and 20 were selected,and their interfacial tension and emulsification properties were tested.The parameters of radial distribution,density distribution,mean azimuth shift,rotation radius and binding energy were analyzed by using MS software，and conducted indoor simulated oil displacement experiments.The results show that with the increase of the number of carbon atoms of long alkyl chain,the oil displacement performance of surfactants was first enhanced and then weakened,and the chemical displacement performance of C₁₆HBC surfactants in betaine was the best.The effects of betaine solution with different alkyl carbon chain number on its energy,radial distribution,density distribution, mean orientation shift, binding energy and other parameters were determined by molecular simulation studies.Simulation oil displacement experiments verified that C₁₆HBC surfactant could significantly improve oil recovery by 20.7%.