Objective: To investigate the association between mobile phone addiction and career maturity among senior high school students, and to examine the mediating role of learning motivation problems and the moderating effect of grade level, so as to provide evidence for targeted interventions to promote career development. Methods: From August to November 2025, a total of 1 591 students from grade 10 to grade 12 were selected from two senior high schools in Haibei Prefecture,Qinghai Province, using cluster sampling. Data were collected using the Mobile Phone Addiction Index, Middle School Students Motivation Scale and Career Maturity Scale. Pearson correlation analysis was conducted to examine the relationships among variables. Hierarchical linear regression was used to explore the association between mobile phone addiction and career maturity. The PROCESS macro (version 5.0 ) was used to test mediation and moderation effects. Results: Mobile phone addiction was negatively correlated with career maturity ( r =-0.21) and positively correlated with learning motivation problems ( r =0.12), while learning motivation problems were negatively correlated with career maturity ( r =-0.23) (all P <0.01). Mobile phone addiction was negatively associated with career maturity（ β =-0.21，95% CI =-0.27 to -0.15）, and learning motivation problems partially mediated the association (indirect effect= -0.03, 95% CI =-0.04 to -0.01)(both P <0.05). The interaction between mobile phone addiction and grade significantly predicted career maturity ( β =0.07, P <0.05). Simple slope analysis showed that the effect of mobile phone addiction on career maturity was strongest among first year senior high school students ( β =-0.26), followed by second year students ( β =-0.19), and was weaker among third year students ( β =-0.12)(all P <0.05). Conclusions Mobile phone addiction is negatively associated with career maturity among senior high school students. Learning motivation problems partially mediate the association, and the association between mobile phone addiction and career maturity is stronger among first and second year students than among third year students.