The invention relates to a bionic leg system for a machine cat type railway bridge erecting machine, which is characterized by comprising a back leg (1), a front leg (2), an auxiliary leg (3), and front and back hydraulic supports (4), wherein the back leg (1) is combined with a girder system of the bridge erecting machine; the back leg (1) is an adjustable rectangular rigid-frame structural member which can be telescoped up and down and turned for 90 degrees to be retracted and narrowed horizontally; the front leg (2) is a portal structure which can be bent and tilted up; the auxiliary leg (3) comprises an upper beam (3a), two upright columns (3b) and a travelling mechanism (3d); the two upright columns (3b) are in a three sleeved column type and are arranged left and right below the upper beam; the travelling mechanism (3d) is positioned below the upright columns; each of the front and back hydraulic supports (4) comprises a four sleeved-column type long upright column (13), a four sleeved-column type short upright column (14) and a lower sleeper beam (16); and the upper end of a jacking oil cylinder (17) installed on the lower sleeper beam (16) is combined with a girder bottom plate of the bridge erecting machine through two detachable sleeved column type lengthening telescoping middle columns (18).