The invention provides an efficient brazing sheet type heat exchanger with externally sheet bars connected in parallel and in series. The efficient brazing sheet type heat exchanger comprises an end plate, sheet bars I, sheet bars II, sheet bars III, a bottom plate I or a bottom plate II; four holes are formed in each of the sheet bars I, and the sheet bars I are tightly welded in parallel, so that the sectional area of a runner of the heat exchanger is increased; only two holes are formed in the end plate, each of the sheet bars II, each of the sheet bars III and the bottom plate; the sheet bars II and the sheet bars III are welded along the sheet bars I, so that the sheet bars I are uniformly distributed; the sheet bars II are connected to the following sheet bars I and the front and back portions in series, the more the sheet bars connected in series are, the longer the runner is; the temperature of a refrigerant which flows out is only 0.5 DEG C lower than that of a hot agent which flows int. The efficient brazing sheet type heat exchanger provided by the invention is small in size, low in price and more thorough to recover waste heat, and can be widely applied to hot water waste heat recovery of occasions such as a single shower room of a family and a public place, a hand washing sink, a bowl and vegetable washing tank, a hairdressing and beauty hair-washing bed and the like. Matched with a small water heater in use, the power of the water heater can be reduced, and the efficient brazing sheet type heat exchanger is energy-saving, remarkable in economical benefit and convenient to popularize and apply.