The invention relates to an enclosing purlin comprising a square steel pipe and light concrete for supporting a foundation pit. The enclosing purlin comprises the square steel pipe, light concrete, an end plate and support ribs, and replaces the original steel enclosing purlin, so as to reduce the consumption of steel material and cost. No cavity is formed in the light concrete, so as to enlarge partial rigidity and effectively avoid partial deformation; the end plate is used for sealing and connecting the enclosing purlin, so as to ensure better integrity of the whole support structure of the foundation pit; and the support ribs are used for preventing excessive deformation of a flange plate and prolonging the service life of the light concrete. During construction, a triangular bracket (small bracket) is erected, then an external member of the enclosing purlin is mounted, and finally, a steel bracing strut is erected and is connected with the enclosing purlin through the flange plate so as to be prevented from falling down. The enclosing purlin has the advantages of convenience in connection, high economical efficiency, reutilization, guaranteed rigidity, and excellent application value.