An injection molding machine includes a lock nut open-close mechanism provided on a moveable platen (1) to engage with at least two tie bars (3), and comprises a pair of rods (23, 24) connecting two pairs of lock nuts (21, 22), and a clamping unit (2). The clamping unit comprises an execution element (25) and a pair of connecting bars for open-close action (26,27) symmetrically arranged. A first end of a first connecting bar (26) is connected to the second nut (212) of the first pair of lock nuts (21), and a first end of the second connecting bar (27) is connected to the first nut (221) of the second pair of lock nuts (22). The second ends of the first and second connecting bars are connected to each other and are driven by the execution element (25) simultaneously to open or close the pairs of lock nuts simultaneously.