Abstract: The cableless intelligent multi-layer water injection technology has the advantages of a simple pipe string and the ability to achieve large displacement in small-sized wellbores. However, the existing pressure wave-based cableless intelligent multi-layer water injection technologies have problems such as slow pressure encoding rate and low regulation efficiency. Therefore, a key technology for efficient cableless multi-layer water injection based on sound wave transmission was proposed. By comparing the finite element simulation results with the actual test results, the influence of the bridge-channel structure and the tubing collars on the sound wave transmission was quantitatively evaluated. The optimal sound source frequency (100–120 Hz) and the range of sound source level were selected. The multi-carrier frequency shift keying (MFSK) modulation method was developed, combined with orthogonal frequency division multiplexing technology, to enhance the signal anti-interference capability. The pool tests and field tests have shown that at the optimal sound source frequency, when the MFSK modulation method is used, and the signal-to-noise ratio is greater than −2 dB, the bit error rate is less than 10⁻⁴. The maximum deviation of the signal-to-noise ratio between the measured values of the test well and the simulated signal was only 2.19 dB. Research has shown that the key technology for efficient cableless multi-layer water injection based on sound wave transmission provides an efficient and reliable solution for the intelligent water injection in offshore oilfields, significantly enhancing the control efficiency and adaptability of multi-layer water injection.