The invention discloses a truss combined beam and upper and layer connected light steel structure and a construction method thereof. The light steel structure comprises upper wall sheet columns distributed at intervals, lower wall sheet columns distributed at intervals, upper wall sheet column lower connecting pipes connected among the lower parts of the upper wall sheet columns, lower wall sheet column upper connecting pipes connected among the upper parts of the lower wall sheet columns, and truss combined beams connected between the upper wall sheet columns and the lower wall sheet columns through upper and lower connectors, wherein the upper and lower connectors are beam passing buckles, the web plates of the beam passing buckles consist of upper web plates and lower web plates, a square connecting port is formed between the upper web plates and the lower web plates, upper chord member connecting plates are extended from the left and right sides of the square connecting ports, and the upper chord member connecting plates and side plates of the beam passing buckles are positioned in the same plane. Through the light steel structure, the structural stability of a light steel keel house system is enhanced, and the floor surface beams, the connecting pipes and the floor panels do not easily vibrate.