The invention discloses a movable die frame girder device suitable for the construction of a widening box girder with large width, which is characterized by comprising a triangle support frame, wherein the triangle support frame and the die frame girder form an integral, and the triangle support frame is connected with a template cross beam; the triangle support frame comprises triangle support frame horizontal rods and triangle support frame slanting rods which are fixedly connected through connecting pins; the triangle support frame horizontal rods and girder upper lug plate seats are fixedly connected through connecting pins; the triangle support frame slanting rods and girder lower lug plate seats are fixedly connected through connecting pins; a vertical jack on the triangle support frame supports and connects the template cross beam and the die frame girder to form an integral. According to the application of the invention, through the utilization of the device, the lateral stiffness and bearing capacity of the movable die frame can be improved, and the construction requirements of the widening box girder with the large width are satisfied, under the condition of not increasing the cross beam fracture surface size.