Vietnam is in the innovation process of its curriculum, contents, and teaching methods, in which integrated teaching is focused on implementation at all levels of education. Therefore, it is necessary to study the factors affecting the effectiveness of subject teaching. This study aims to determine the main factors affecting the integrated teaching effectiveness of secondary school math teachers. Simultaneously, it proposes some solutions to improve math teaching quality. SPSS 26.0 software was used to support data analysis from a survey of 508 math teachers at 314 secondary schools in Vietnam. The results of seven analyzed variables show a strong correlation between them. Of the seven variables, six affect teachers' effectiveness of integrated teaching in secondary school math. The most influential factor is the teacher's perception of integrated teaching, followed by objectives and contents of secondary school math; salary, bonus, and remuneration policies for teachers; teaching environment; exploitation of learning materials and use of technology; and facilities. The study also shows that seniority, educational qualifications, and teaching locations do not significantly affect the effectiveness of integrated teaching. However, there is a difference between teachers and school administrators in the effectiveness of integrated teaching. The new findings of this study are useful references for math teachers and administrators in improving the quality of integrated teaching in secondary schools, teacher training institutions in developing training programs and learning outcomes, and policymakers in developing strategies to promote teaching effectiveness in secondary schools.