Objective? To detect copy number variation sequencing (CNV) in children diagnosed with developmental delay (DD). Methods? Clinical data of children diagnosed with DD in Shanxi Provincial Children's Hospital (Shanxi Maternal and Child Health Hospital) from 2017 to 2019 were collected, and all patients had karyotype tested. CNV-seq was used to detect CNVs in these patients. ClinVar, DECIPHER, OMIM, DGV and other databases were used as references for data annotation, and the pathogenicity of CNV is classified according to ACMG guideline. Related articles were retrieved through the PubMed database. Results? Ninety patients were tested and the diagnostic rate was 20% ( 18/ 90 ). In which 15 patients were found have pathogenic CNVs,and three patients were found have likely pathogenic CNV. In addition, 32 patients carried copy number variation of unknown significance. Conclusion? This study confirmed that CNVs were an important cause of DD, and the CNVs > 1Mb in length are more likely pathogenic. In view of this, we recommend that CNVs testing be the first-line test in children with DD. CNV-Seq is a more efficient CNVs detection method based on high-throughput sequencing.