Gastrointestinal stromal tumors (GISTs) are one of the most common mesenchymal tumor types and usually contain KIT or PDGFRA mutations. GISTs with concomitant low- and high-grade components are seen in clinical practice. Herein, we retrospectively analyzed the histological characteristics and immunohistochemical results of 22 GIST cases with concomitant low- and high-grade tumors. Whole-exome sequencing (WES) was performed on ten pairs of high-grade GIST specimens and matched low-grade samples. Differential oncogenes mutated only in high-grade GISTs were identified, which were confirmed by Sanger sequencing. Fluorescence in situ hybridization was employed to detect MYC copy number variation. High-grade GISTs were more likely to have lower CD34 expression and a higher Ki-67 proliferation index compared to the matched low-grade tumors. WES identified 30 differential cancer-associated genes mutated only in high-grade GISTs; Sanger sequencing confirmed ten relevant differential oncogenic mutations in nine genes (MGA, ARID1A, LATS2, MAX, PIK3CA, RB1, RPS6KB2, SDHA, and SETD2). Two patients had MGA mutations, whereas other gene mutations occurred in only one patient. Most of the differential cancer-associated genes are mainly involved in cell cycle control. MYC copy number gain was a common genetic variation. High-grade GISTs revealed more MYC copy number gains than matched low-grade tumors, and low-grade GISTs with coexisting high-grade components showed more MYC copy number gains than pure low-grade GISTs. Moreover, MYC copy number gain was positively correlated with the mitotic index and Ki-67 proliferation index. Alterations in cell cycle regulation-associated genes, such as genetic mutations and MYC copy number gain, may promote primary progression from low-grade GISTs to high-grade tumors by regulating tumor cell proliferation.