The invention relates to a cast-in-site watery concrete stand template and its constructing method, where the seat platform and seat-back stile are encircled with seat platform bed template panel, seat back stile's up-front vertical surface hanger template, seat back stile's down-front vertical template panel, rear vertical template panel and seat back stile's bed template panel, and the position of the ventilating hole is pre-buried with PVC pipe; the external angle position is equipped with an circular-arch chamfered external angle die strip plate; a triangular strip is cut off outside the bottom end of the seat back stile's up-front vertical surface hanger template in the internal angle position; horizontal secondary keel and vertical primary keel are used to support the back of each template panel; the primary keel is supported on support beam, the support beam is arranged on the support beam body; the primary keel of the hanger template hangs and is supported by temporary vertical support, and wall-passing bolt runs in order through the primary keel of the hanger template, seat back stile's up-front vertical surface hanger template, ventilating hole, rear vertical template panel and the primary keel of the rear vertical template panel and is fastened. And the invention advances constructing schedule, assures construction quality and besides, reduces engineering cost.