Ocean waves are one of the most important and complex elements in ocean hydrology. Effective monitoring of ocean waves is crucial for various applications, including nearshore production, marine scientific research, and the prediction of undersea earthquakes. In this paper, a pressure-type ocean wave height sensor based on Fiber Bragg Grating (FBG) is designed for nearshore wave height measurement. The sensor's pressure measurement principle is based on the elastic diaphragm made of beryllium bronze. The external seawater pressure acts directly on the elastic diaphragm, which makes the center of the diaphragm change in deflection. Then it changes the effective length of the Pressure Fiber Bragg Grating (P-FBG), leading to the blue shift of its center wavelength, and detects the seawater pressure through the measurement of the wavelength change amount. For temperature compensation, a Temperature Fiber Bragg Grating (T-FBG) is positioned within the same cavity as the Pressure-Measuring Grating (P-FBG). The T-FBG is solely sensitive to temperature. By correlating the center wavelength drifts of the P-FBG and T-FBG through a dedicated formula, the influence of temperature variations on pressure measurements can be dynamically compensated, thereby enhancing measurement accuracy. Utilize the relationship between the underwater pressure wave and the surface wave height to achieve compensation of the wave pressure value, and then calculate the wave height through the upper spanning zero method. The optimal design parameters of the sensor are determined by combining previous laboratory research and simulation analysis. A finite element simulation analysis is conducted after constructing the sensor model to verify its feasibility. Three sinusoidal pressure signals with different amplitudes and periods are used to simulate small, medium, and large waves in the real ocean environment. The simulation results demonstrated that the elastic diaphragm exhibited good stability and responsiveness under the three types of positive pressure signals, with a response time of only 3.8 ms, significantly lower than the actual wave collection frequency, indicating that the response time had a negligible impact on measurement results. When the external ambient temperature changed abruptly from 15 ℃ to 20 ℃, the simulation results indicated that the average absolute temperature difference between the P-FBG and the T-FBG is only 0.03 ℃, suggesting that both experienced the same temperature change. Performance calibration experiments for pressure and temperature are conducted on the FBG ocean wave height sensor, revealing that the P-FBG wave sensor had a sensitivity of -9.486 nm/MPa, a linear correlation coefficient of 0.999 97, and a pressure resolution of 0.000 1 MPa (water depth resolution is 1 cm) within the pressure measurement range of 0 to 0.3 MPa. The temperature sensitivities of the P-FBG and the T-FBG were 24.9 pm/℃ and 30.6 pm/℃, respectively, with resolutions better than 0.04 ℃ and a linear correlation coefficient of 0.999 87 within the temperature measurement range of 0 to 35 ℃. In the hydrostatic test, the measured water level fitted the actual value with a degree of fit of 99.919%. The calibration tests confirmed that the FBG ocean wave height sensor can achieve high-precision wave measurement. This paper proposes a zero-line selection method based on the Smoothed Prior Approach (SPA), specifically designed to address the characteristics of measured ocean wave data. The proposed method effectively resolves the zero omission issue and exhibits superior applicability compared to conventional approaches. A five-day field test of the FBG ocean wave height sensor and the SBF3-2 wave buoy was conducted at the Luhaifeng Ocean Ranch. The results showed that the sensor and the SBF3-2 wave buoy obtained the same trend in wave height values, with a correlation coefficient of more than 0.85. The proposed pressure-type fiber grating ocean wave height sensor features underwater passivity, anti-electromagnetic interference, and fast signal transmission, offering a novel optical measurement method for nearshore wave height measurement and demonstrating promising application prospects.