Nutrient inputs through submarine groundwater discharge (SGD) play a significant role in sustaining primary productivity and nutrient cycling in coastal areas. Currently, various geochemical isotopes are used to trace the SGD processes. However, mass balance models of stable water isotopes (²H and ¹⁸O) are seldom used in SGD estimating. In this study, mass balance models of ¹⁸O and radium isotopes were used to estimate the water age and SGD in Laizhou Bay, China based on the isotope data sampled in August 2017. The water age ranged from 23.5 to 50.0 days with an average of 32.1 ± 16.3 days. The SGD flux ranged from 1.29 × 10⁸ m³ d^-1 to 2.84 × 10⁸ m³ d^-1 with an average of (2.07 ± 1.04) × 10⁸ m³ d^-1. The sensitivity analysis revealed that estimated results of the water age and SGD are very sensitive to the ¹⁸O value in evaporation, as well as ¹⁸O and ²²⁸Ra values in groundwater end-members. Based on the isotope method, the proportion of the Yellow River discharging into Laizhou Bay was estimated to be less than 27% of the total discharge. Furthermore, combining water and salt mass balance models, the submarine fresh groundwater discharge (SFGD) flux ranged from 0.54 × 10⁷ m³ d^-1 to 1.31 × 10⁷ m³ d^-1 with an average of (0.93 ± 0.46) × 10⁷ m³ d^-1. This study reveals that stable and radium isotopes can be effectively combined to estimate the water age and SGD, which may be applied to coastal areas elsewhere.