Significant wave height (SWH) is an important parameter to describe the sea state and plays an important role in the prediction of wave and ocean dynamics. However, traditional methods such as satellite radar altimeter and buoy are difficult to achieve high spatial-temporal resolution SWH estimation. As a novel remote sensing technology, spaceborne Global Navigation Satellite System-Reflectometry (GNSS-R) provides a new opportunity to estimate SWH. This paper presents a new method for retrieving SWH of spaceborne GNSS-R waves based on normalized integral delay waveform (NIDW). Firstly, the spaceborne GNSS-R delay Doppler maps (DDMs) are denoised and filtered in order to strictly control the quality of DDM data. Then, NIDW is extracted from DDM, and four GNSS-R observables are calculated based on NIDW (i.e. leading edge slope of NIDW, trailing edge slope of NIDW, leading edge waveform summation of NIDW and trailing edge waveform summation of NIDW). Subsequently, based on these four observables, an empirical SWH retrieval model of spaceborne GNSS-R is established. Finally, ERA5 and AVISO SWH data products are used as validation data respectively, and the SWH estimation results of the retrieval model are compared and analyzed with ERA5 and AVISO SWH data products. The experimental results show that when the EAR5 SWH data is used as the verification data, the root mean square error (RMSE) and correlation coefficient (CC) of the four observables are better than 0.66 m and 0.65, respectively. When the AVISO SWH data is used as the verification data, the RMSE and CC of the four observables are better than 0.68 m and 0.70, respectively. It further shows that the modeling method proposed in this paper is feasible and reliable in spaceborne GNSS-R SWH estimation.