Teachers' professional development (PD) is considered to be a topic of interest in science education. This study examines the impact of professional development in science pedagogy (PD in PED) on students' achievement and how it influences teachers' instruction based on the Trends in International Mathematics and Science (TIMSS) 2019. The sample comprised 2,968 fourth-grade students and their 145 teachers and 3,265 eighth-grade students and their 150 teachers in Hong Kong. Teachers' emphasis on science investigation (TESI) was included as a mediating factor. Multilevel mediation analyses revealed that PD in PED was significantly positively related to students' science achievement, while the link between PD in PED and students' science achievement was positively and completely mediated by TESI in the fourth grade. However, TESI had no mediating effect in the eighth grade, a finding that may be attributed to the characteristics of teacher PD programmes in different grades. The results also show that many teacher PD programmes are not of high quality and are intellectually superficial from the perspective of students' achievement gains. Implications for teacher PD practices are discussed as well.