A displacement difference solution density measuring apparatus includes: a first floater (1) and at least two second floater (2) in an inner cavity of the first floater (1). The first floater is provided with a coil (4). The second floater (2) is provided with an iron core (3). The iron core (3) passes through the coil (4) and can move within the range included by the coil (4). A processing circuit and emitting apparatus (5) is connected to the coil (4) for emitting a detection signal to a receiving apparatus. A method of measuring solution density with the apparatus includes the steps of: 1) detecting the change quantity of inductance or capacitance of the coil to obtain the height difference [delta]h between the two floaters; and 2) acquiring the density [rho] of the to-be-metered solution according to the relationship among the solution density [rho], mass W1 and cross section area S1 of the first floater, and mass W2 and cross section area S2 of the second floater, and the height difference [delta]h between the two floaters. The apparatus and the method are suitable for sampling and on-line detection of densities of various solutions, and especially are suitable for on-line detection, monitoring or remote-detection of concentration of solutions in a place being inconvenient to sampling and dangerous. The apparatus is high in detection precision and simple in structure.