  We develop an analytical approach, verified by the nice agreement with the
numerical relativity simulation results, to calculate the quasinormal mode
spectrum of the Kerr-Newman black hole. Then we analyze the gravitational wave
data with the ringdown waveform model including both the fundamental mode and
the overtone modes, and find that it can efficiently constrain the charge of
the source. For GW150914, the charge-to-mass ratio of the remnant black hole is
constrained to be $\leq 0.38$ at $90\%$ credibility. Our waveform model can be
widely applied to other GW150914 like events. With the sole ringdown data, it
is capable of constraining the electric, magnetic, or other $U(1)$ dark charges
carried by black holes, as well as the deviation parameter ($\alpha$) of the
scalar-tensor-vector gravity. Indeed, a constraint of $\alpha\leq 0.17$ is
achieved with the ringdown data alone for the first time.
