Background: PARP inhibitors (PARPi) have been widely used in the maintenance therapy of ovarian cancer (OC) and have shown strong anti-tumor effect in tumors with homologous recombination defects (HRD). However, there is still a lack of systematic clinical data on drug resistance mechanisms and tumor microenvironment changes. Methods: 13 OC patients who were treated with PARPi at West China Second University Hospital were enrolled. PARPi treatment-naive samples (PNS, n = 11) and post-PARPi progression samples (PPS, n = 13) were acquired. Two next-generation sequencing (NGS) panels (Master panel, 563 genes for exons of DNA plus 1831 genes for RNA; HRD panel, 70,000 SNPs for HRD score evaluation, AmoyDx) were used to analyze gene variation, expression, and HRD score changes in tumors. Results: Median duration of treatment for PARPi was 17 months (9-36), and the median lines of chemotherapy before PARPi was 2 (1-3). 38.5% (5/13) patients achieved R0 resection. BRCA reversion mutations were observed in 30.0% (3/10) of the gBRCA+ PPS, including 1 splicing variant and 2 frameshift mutations. High-frequency variants also included MYC amplification (30.8%, 4/13) and ZFHX4 mutations (23.1%, 3/13); Amplification of ch11 q13 were observed in two patients (15.4%). HRD scores and tumor mutation burden (TMB) were significantly increased in PPS. RNA expression profiling showed that the expression of NGFR, CCL18, C7 and other 85 genes in PPS was up regulated. GO/KEGG analysis showed that NF-κb pathway, immune response related cell activation, cytokine - cytokine receptor interaction, B cell receptor signaling pathway, NK cytotoxicity pathways, and etc., were significantly upregulated in PPS, suggesting that PARPi may activated immune-related pathways of the tumor microenvironment (TME). Downregulated pathways in PPS include JAK-STAT signaling pathway, Wnt signaling pathway, and etc. In PPS, the level of memory B cells and neutrophils were significantly up-regulated, while the levels of immune suppression by myeloid cells were significantly down-regulated. Conclusions: Our small-scale study confirmed that BRCA reversion mutation is one of the important resistance mechanisms of PARPi, and the reversion mutation is located near the germline pathogenic mutation to counteract the reading frame change caused by the frameshift mutation. The up-regulation of B-cell receptor, NK cytotoxic cytokine receptor interaction and other related pathways suggested that PARPi may activate immune response, and the changes in the levels of memory B cells, C7 and neutrophils also confirmed the results above. Epithelial mesenchymal transition induced by CCL18 may also lead to PARPi resistance and activate the NF-κb pathway simultaneously, increasing the inflammatory characteristics of TME. These findings can provide ideas for future treatment, and larger samples are needed to verify later.