We consider a dynamic inverse problem for a dynamical system describing the propagation of waves in a Krein string. We reduce the dynamical system to the integral equation and consider the important special case when the density of a string is given by nite number of point masses distributed on the interval. We derive the Krein-type equation and solve the dynamic inverse problem in this particular case. We also consider the approximation of constant density by point-mass densities uniformly distributed on the interval and the effect of appearing of the finite speed of a wave propagation in the dynamical system.