The inhibition effects of water on the conversion and coking of olefins on SAPO-34 were investigated on fixed bed reactor. Remarkable reduction effects of water on the conversion and coking of ethene, ethene + propene and ethene +1-butene were observed. The conversion and coking of propene and 1-butene were also inhibited, though less noticeable. The co-feeding of water strongly suppressed the formation of insoluble coke and had little effect on the formation of soluble coke. It is supposed that the conversion and coking of olefins is initiated with the oligomerization and alkylation of olefins which lead to the formation of long intermediates. Then these long molecules undergo cracking to form small species or cyclization and aromatization to form confined cokes. Water competes with olefins for the accessing of acidic sites and leads to desorption of olefins, which reduces the formation of long intermediates. Ethene can be easily desorbed while water is co-fed. The oligomerization and alkylation of ethene is suppressed greatly by water which reduces intermediates for the following cracking, cyclization and dehydro-aromatization, thus the conversion and coking of ethene-rich streams were remarkably reduced. The effect of water on the adsorption of propene and 1-butene is much weaker than that on the adsorption of ethene. Therefore, some inhibition effects of water on the conversion and coking of propene and 1-butene was observed, but much less significant than that of ethene. The role of water in MTO process can be well interpreted with the inhibition effects of water on further conversions and coking of olefins.