Objective To observe the effect of bone marrow mesenchymal stem cells (BMSCs) conditioned medium on microglia (MGs) and its secretion of arginase 1 (Arg1). Methods The BMSCs separated through differential adhesion method from the femur and tibia marrow of 4-week-old Sprague Dawley (SD) rats were cultured and identified by Vimentin immunofluorescence staining; whereas MGs separated through trypsin digestion method from the brain of 3-day-old SD rats were cultured and identified by Iba1 immunofluorescence staining. The primary MGs were cultured with DMEM/F12 medium containing BMSCs conditioned medium (experimental group) and with single DMEM/F12 medium (control group), respectively. After 48 hours of culture, the morphology of MGs was observed by inverted phase contrast microscope, the activated state of MGs was detected by using Iba1 immunofluorescence staining, and Arg1 expression of MGs was assessed by Iba1-Arg1 double-labelling immunofluorescence staining and Western blot method. Results Inverted phase contrast microscope observation showed that BMSCs entered logarithmic growth phase at 14 days after culture, and more than 98% cells were positive to Vimentin immunofluorescence staining; whereas MGs entered logarithmic growth phase at 21 days after culture, and around 80% cells were positive to Iba1 immunofluorescence staining. Inverted phase contrast microscope observation displayed that in the experimental group, MGs were activated with increased size of soma, shortened process, and amoeba change. Immunofluorescence staining displayed that the Iba1 positive cells number in the experimental group was significantly higher than that in the control group (t=0.007, P=0.000); double-labelling immunofluorescence staining revealed that the Iba1-Arg1 positive cells number in the experimental group was significantly higher than that in the control group (t=0.007, P=0.000); and Western blot results elucidated that the relative expression of Arg1 protein in the experimental group was significantly higher than that in the control group (t=0.001, P=0.000). Conclusion BMSCs conditioned medium can activate MGs and induce MGs to express Arg1.