In order to improve the laser acoustic communication technology between air platforms and underwater objects, the n-multifrequency shift keying (n-MFSK) modulation method was proposed to further enhance the laser acoustic air-water cross-media communication rate. Based on the thermal expansion effect of laser acoustic, the modulation methods of the long-pulse-duration laser method and the high repetitive rate method were defined and simulated. The result was obtained that 2-MFSK modulation doubles the communication rate on top of 2-FSK modulation for a modulation frequency number of 2. The n-MFSK modulation was achieved by the long-pulse-duration laser method, mainly through the laser array by means of frequency superposition, and the high repetitive rate method by controlling the allocation of time periods on the time domain of the laser frequency change. The results show that as the number of modulation frequencies increases, n-MFSK modulation can increase the communication rate and improve the band utilization relative to n-frequency shift keying (n-FSK) modulation at the cost of loss of the sound pressure level (SPL) and in-water-range. This study provides a reference for the practical application of laser acoustic air-water cross-media communication in the future.