This study focuses on mid-ocean ridge basalts (MORBs) collected near 5.3°S of the East Pacific Rise (EPR). In-situ electron microprobe (EPMA) combined with petrogeochemical analysis for the zonal structures, compositional variations of plagioclase phenocrysts and whole rocks were used to reveal magma evolution processes. The basalts in the study area exhibit typical characteristics of normal mid-ocean ridge basalts (N-MORBs) with a relatively high average partial melting degree (15%~20%) of source mantle. The basalts possess a typical tholeiitic texture, and most of the plagioclase phenocrysts display various types of zonal structures, which record multiple magmatic pulsation or mixing events. Backscattered electron (BSE) images and compositional variations of plagioclase phenocrysts indicate that basalts contains three types plagioclase phenocrysts. Type a is characterized by a simple reverse zonal structure, recording one magmatic mixing event. Type b exhibits a sparse rhythmic zonal structure, documenting two magmatic mixing events. Type c with large size has obvious zonal structures, which may formed in a magmatic environment at the bottom of the magma chamber continuously replenished by mantle melts. Based on the plagioclase geobarometer, the preliminary estimation shows that the magma chamber is approximately located 11.44~13.39 km below the mid-ocean ridge.