In this paper,the equation of forced vibration of the undamping strings driven by a periodic driving force was established by means of Dirac δ function. The equation is solved with boundary conditions. The solution is shown and the influence of frequency on amplitude was analyzed. Resonance, antiresonance and pseudoresonance were found, and the conditions of the three phenomenons are determined. At last, the experiments on verifying the conditions of resonance and pseudoresonance are designed, the experiment results are consistent with theoretical predictions．