Gassy soil is prevalent in coastal regions, and the presence of gas bubbles can significantly alter the mechanical properties of soil, potentially leading to various marine engineering geological hazards. In this study, a series of triaxial tests were conducted on fine-grained gassy soils under different consolidation pressures (p_c’), stress paths, and initial pore water pressures (u_w0). These tests were also used to verify the applicability of a newly proposed constitutive model. According to the test results, the response to excess pore pressure and the stress–strain relationship of fine-grained gassy soils strongly depend on the initial pore water pressure (u_w0), with the degree of variation being influenced by the consolidation pressure (p_c’) and stress path. As u_w0 decreases, the undrained shear strength (c_u) of fine-grained gassy soils gradually increases, and this is lower under the reduced triaxial compression (RTC) path compared to the conventional triaxial compression (CTC) path, which can be attributed to the destruction of the pore structure due to an increase in gas volume. The newly proposed model accurately predicts the pore pressure and stress–strain relationship of fine-grained gassy soils at low consolidation pressures (p_c’), but it falls short in predicting the mechanical behavior during shear progression under high p_c’ or the RTC path. Although the model effectively predicts the excess pore pressure and deviator stress at the shear failure point (axial strain = 15%), further improvement is still required.