This study was conducted to investigate the influence of μ-opioid receptor gene (OPRM1) A118G polymorphism on dosage of morphine in advanced liver cancer patients. Seventy patients with advanced liver cancer at Changyi People's Hospital, Shandong Province, China, were included from February 2019 to December 2020. The dosage of morphine in patients with OPRM1 A118G different genotypes was compared. Thirty patients (42.86%) were of AA genotype, 35 (50.00%) were AG genotype and 5 (7.14%) were GG genotype. There was a significant difference in morphine dosage within the first 24 hours in patients with AA, AG and GG genotypes (all p <0.001), and morphine dosage in patients with GG genotype was the highest. In conclusion, OPRM1 A118G genotype may affect the dosage of morphine in advanced liver cancer patients. More dosages of morphine are needed for pain control in patients with G allele.