Recently on the December 4, 2021 at 03:00 PM, 3676 m high Mount Semeru located in the East parts of Java Island has erupted. To our best knowledge, an immediate and rapid systematic analysis of the volcanic ash plume courses, PM_2.5 emissions, and environmental impacts based on Mount Semeru eruption has not been implemented so far. Then, this research aims to provide and fill the research gap on the rapid assessment of recent Mount Semeru eruption. From the result, it is clearly visible that for 12 hours the volcanic ash plume course was eastward. The volcanic ash plume can travel a distance of 0–10 km to the North and South directions, and more than 10 km to the East direction. The size of the volcanic ash plume was large at 02:00 AM on December 5, 2021. The smallest size of a volcanic ash plume was recorded at 09:00 PM on December 4, 2021. Most parts of the ash plume (55.98%) or equals 39.01 km² contain fallen volcanic material amounts ranged from 1 kg/m² to 10 kg/m². The fallen volcanic material amount peaked between 08:00 PM and 11:00 PM. Based on the estimation, the PM_2.5 content in the atmosphere increased after the eruption. The mean of PM_2.5 before the eruption was 48.5 ± 19.3(95%CI: 29.2 to 67.8 ug/m³). While after eruption the mean of PM_2.5 was 79.4 ± 32.2(95%CI: 47.2 to112 ug/m³). It indicated that the Mount Semeru eruption has increased the PM_2.5 equals 63.65%.