The invention provides a catalyst for benzene absorption liquid-phase alkylation of high-concentration ethylene, and a preparation method thereof. The preparation method comprises the following steps:uniformly mixing a silicon source, an aluminum source, an inorganic alkali, a microporous template, deionized water and sodium carboxymethylcellulose (CMC), and directly carrying out high-temperaturecrystallization, solid-liquid separation and ion exchange to obtain an H-Beta (CMC) molecular sieve; and then mixing and kneading a certain amount of the H-Beta (CMC) molecular sieve and a binder, and drying and calcining the obtained mixture to prepare an H-Beta (CMC) A catalyst. The catalyst is applied to a process for benzene absorption liquid-phase alkylation of high-concentration ethylene. The high-concentration ethylene contains 60-99 v% of ethylene, 0.1-0.5 v% of hydrogen, 0.1-0.5 v% of nitrogen, 0.1-0.2 v% of COx, 0.1-0.5 v% of methane, 0.1-25 v% of ethane, 0.1-20 v% of propane and 0.1-20 v% of butane. Compared with conventional catalysts, the catalyst prepared by the method has a higher stability on benzene absorption liquid-phase alkylation of high-concentration ethylene.