A method for forming a plurality of beams connected in series is provided. The method comprises the following steps: providing a plurality of columns; providing a plurality of pre-assembled bar combinations, wherein each of the plurality of pre-assembled bar combinations comprises a plurality of lower main bars and a plurality of lower stirrups, and at least one end of the plurality of the lower main bars has connection sections extending beyond the plurality of lower stirrups; hoisting each of the bar combinations so that the two ends thereof are placed on top of two adjacent columns of the plurality of columns and the connection sections of the plurality of lower main bars of adjacent plurality of bar combinations overlap; and connecting the plurality of connection sections of the plurality of lower main bars of the adjacent plurality of bar combinations.