The invention relates to a method without a circulation loop for producing synthetic natural gas by complete methanation of synthesis gas. Conversion reaction is performed on one part of crude synthesis gas to obtain hydrogen-rich crude synthesis gas, direct cooling is performed on the other part of the crude synthesis gas, liquid water is separated, and then carbon-rich crude synthesis gas is obtained; impurity gas is respectively removed by purification treatment; the hydrogen-rich synthesis gas or the carbon-rich synthesis gas after purification enters a first-stage methanation reactor as a reaction stream to perform methanation reaction, the carbon-rich synthesis gas or the hydrogen-rich synthesis gas is taken as a control stream for controlling the reaction depth of other stages of methanation reactors, and a reaction product is taken out of the first-stage methanation reactor, cooled and subjected to supplementation of part of the control steam and then enters the next-stage methanation reactor till all of the control stream is supplemented into the reaction stream before the last-stage methanation reactor; and the reaction product which is rich in methane enters a synthetic natural gas post-treatment unit to obtain a synthetic natural gas product. The method disclosed by the invention has the advantages of saving investment and energy consumption and being easier to control and operate.