microRNAs can cause male infertility by impacting sperm quality and impaired spermatogenesis. Since the miR-125 family plays an important role in regulating embryo development, but the function of miR-125b-2 in male reproduction remains unknown. In this study, we prepared a model of miR-125b knockout (KO) mice. Among the KO mice, the progeny test showed that litter sizes decreased significantly and the rate of non-parous females increased significantly (p<0.05). At the same time, the testosterone concentration increased significantly (p<0.01), with the remarkable decrease for estradiol (p<0.05). Moreover, sperm count decreased obviously (p<0.05) and the percentage of abnormal sperms increased significantly (p<0.01). Testicular transcriptome sequencing demonstrated that there were 173 up-regulated genes, including Papolb (PAP), and 151 down-regulated genes in KO mice compared with wild type (WT). KEGG and GO analysis showed many of these genes were involved in sperm mitochondrial metabolism and other cellular biological processes. Meanwhile, the sperm mitochondria DNA (mtDNA) copy number was increased significantly (p<0.01) in KO mice, but the integrity of mtDNA and nuclear DNA (nDNA) had no change. In the top 10 up-regulated genes, as a testis specific expressing gene, PAP can affect the process of spermatogenesis. Western blotting and Luciferase Assay validated that PAP was the target of miR-125b-5p. Intriguingly, we also found that both miR-125b and PAP were only highly expressed in germ cells (GC) instead of Leydig cells (LC) and Sertoli cells (SC), and miR-125b-5p could target PAP to regulate TM3 cell secretion of testosterone (p<0.05). Our study firstly demonstrated that miR-125b-2 could regulate testosterone secretion by directly targeting PAP and increase sperm mtDNA copy number to affect semen quality. The study indicated that miR-125b-2 had a positive influence on the reproductive performance of animal and could be a potential therapeutic target for male infertility.