The invention discloses a method for measuring the activity of transmethylase in real time and a kit. The method comprises two reactions carried out at the same time in a 
reaction system; according to the first reaction, in a buffer 
system where it is guaranteed that S-
ademetionine is adopted as a 
methyl donor and the transmethylase has the 
biological activity, a sample containing the transmethylase (MT), the 
methyl donor and a corresponding substrate are added, or the S-
ademetionine methyl donor and the corresponding substrate are directly added into the liquid sample containing the transmethylase, and a reaction is carried out so that an S-adellosyl 
homocysteine (SAH) product can be generated, wherein the substrate is 
receptor matter of methyl; according to the second reaction, an immunological method is adopted for detecting the content of SAH to determine the activity of the transmethylase in the 
reaction system. The biochemical reaction of MT 
catalysis and the immunoreactions for measuring the product SAH are carried out at the same time, the two processes are organically combined, and great significance is especially achieved on accurate measurement of SAH with extremely unstable molecularity.