<p>Gonorrhea and syphilis have a high prevalence and the potential for synergistic transmission. Thus, understanding their interactive transmission dynamics is of significant public health importance. This paper develops a gonorrhea-syphilis coinfection model to analyze their interactive transmission dynamics, addressing a critical gap in understanding how these two sexually transmitted infections influence each other’s spread. Unlike previous studies that focus on single infections, this work uniquely captures the complex interplay between the diseases through rigorous mathematical analysis, establishing the existence and stability of disease-free, boundary endemic, and co-existence endemic equilibrium, as well as bifurcation characteristics. A series of numerical simulations is then conducted to verify the analytical results and illustrate the threshold dynamics governed by the basic and invasion reproduction numbers. Importantly, a case study using reported syphilis and gonorrhea data from China (2004–2020) provides empirical validation, including epidemiological data analysis, fitting of cumulative case numbers from 2011 to 2020, estimation of basic reproduction numbers, and sensitivity analysis of invasion reproduction numbers. The fitting results estimate the basic reproduction numbers for syphilis and gonorrhea over the past decade as 1.1410 and 1.3594, respectively, indicating sustained but moderate transmission. The empirical findings reveal that the risk of coinfection remains limited under current conditions, while the distinct reproduction numbers suggest that differentiated intervention strategies are recommended for the two diseases. This practical conclusion can guide resource allocation and targeted public health policies.</p>