The invention relates to a cleaning machine for an inner cavity of a heat exchanger. The cleaning machine comprises a machine frame, wherein a first water tank and a second water tank are arranged on the lower side of the machine frame, and the first water tank and the second water tank are respectively provided with a liquid inlet, a liquid outlet and an overflow opening; the second water tank is connected with a first filter and the first water tank through a pressure pump, the back end of the first water tank is connected with a circulating pump and a second filter, the back end of the second filter is connected with a first reversing valve and a second reversing valve, the second water tank is connected with a third reversing valve and a fourth reversing valve, the first reversing valve and the fourth reversing valve are connected with an inlet of the inner cavity of the heat exchanger through first workpiece connecting valve lines, and the second reversing valve and the third reversing valve are connected with an outlet of the inner cavity of the heat exchanger through second workpiece connecting valve lines; a plurality of first workpiece connecting valve lines and a plurality of second workpiece connecting valve lines are arranged on the upper side of the machine frame; a water accumulating tray is arranged at a position, positioned on one side, of the first workpiece connecting valve lines and the second workpiece connecting valve lines, of the machine frame; the inner cavity of the heat exchanger is placed on the water accumulating tray. The cleaning machine for the inner cavity of the heat exchanger is thorough in cleaning and high in cleaning efficiency.