Alkali metal modified ZSM-5 (ZSM-5-M, M: Li, Na, K, Cs) was prepared by ion-exchange method. The ion-exchange of alkali metal cations with silanol group occurred along with hydrolysis of the Si-O(H)-Al, leading to increase of Lewis acid, decrease of strong and Bronsted acids. K+ and Cs+ were easier to exchange with proton of -OH compared with Na+ and Li+ due to their smaller hydrated radius, resulting in higher ion exchange capacity for ZSM-5-K and ZSM-5-Cs. Their catalytic performances in supercritical n-dodecane cracking were studied. The gas generation rates (GGR) and heat sinks under 550 degrees C and 4 MPa increased as following: ZSM-5-Cs < ZSM-5 < ZSM-5-K < ZSM-5-Li < ZSM-5-Na. The GGR and heat sink of ZSM-5-Na reached 60.3% and 2826 kJ kg(-1), respectively. Thus moderate attenuation of strong acid and enhancement of basic sites could enhance the catalytic performance in supercritical n-dodecane cracking. The selectivity for light alkene was enhanced and the coke was restrained due to the regulated acid-base properties, since decreasing strong acidic sites and increasing the basic sites could suppress the secondary reactions that consumed light alkene and generated coke.