The invention relates to a high pressure water pump and a steam powered nailing gun having the high pressure water pump. In certain embodiments, high pressure water pump includes: an upper pump body, a lower pump body, a water discharge valve, a water intake valve, a plunger, a guide sleeve, a hammering cap, an adjustment knob, and an evacuation valve. Both upper and lower pump body are connected by connecting bolts. A sealing ring is disposed between upper and lower pump body and a sealing element is disposed between the upper pump body and plunger. Plunger is sheathed in upper pump body. The hammering cap is threadedly connected to the plunger. The adjustment knob is connected to an upper end of upper pump body. The hammering cap is moveably connected to the adjustment knob through the plunger. A plunger reset spring is disposed between hammering cap and adjustment knob.