The invention discloses a method of testing a small hydropower efficiency curve online. Operation condition calculation of a unit is acquired in real time through an online data acquisition device; different operation conditions are classified; the density of test points is increased; and finally, an actual curve of unit efficiency is obtained. In the prior art, a data fitting method is adopted to approximate the relation, and a small hydropower efficiency curve is obtained through efficiency fitting of 8-12 measuring power points, and therefore, the operation condition points except the test power points have certain error, and the unit efficiency at the non-test points cannot be reflected accurately. By using the method of the invention, the problem is solved. The shortcomings of the prior art are overcome.