In order to improve the spectral efficiency and power efficiency for underwater optical-wireless communication (UOWC), a UOWC system based on layered asymmetrically clipped optical orthogonal frequency division multiplexing (Layered-ACO-OFDM) is proposed. The system makes full use of the spectrum resources lost in traditional asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) by the means of layered modulation of subcarriers, which greatly improves the spectrum efficiency for optical wireless communication system. Moreover, since the Layered-ACO-OFDM system does not need to add direct current bias, the power efficiency is also greatly improved. A UOWC channel model based on Monte Carlo method is established considering the absorption and scattering effects of visible light transmission in seawater, and the channel response under different chlorophyll concentrations and different receiver parameters is studied comprehensively. The results show that the UOWC system based on Layered-ACO-OFDM has the advantage of high spectrum efficiency compared with the traditional ACO-OFDM. When the signal-to-noise ratio is 27 dB, it can achieve high-speed communication of 1 Gb/s at a distance of 10 m in clean seawater.