Objective To identify the GNAS gene mutation resulting in pseudohypoparathyroidism type Ⅰa (PHP-Ⅰa) in one patient. Methods The clinical data of a patient with pseudohypoparathyroidism type Ⅰa was retrospectively analyzed. All the 13 exons of GNAS were sequenced using Sanger method for the patient and the parents. The distribution of suspected causal mutation was screened in 478 healthy controls. To clarify the origin of the mutation, we performed targeted high-depth sequencing of GNAS exon harboring the mutation for the patient and the parents. Results The clinical data of the patient with the laboratory results of hypocalcaemia, hyperphosphataemia, elevated serum PTH, together with the features of AHO, conformed to the characterization of PHP-Ⅰa. The sequencing of GNAS exons identified a missense mutation (c.479G>C, p.R160P) located at exon 6 in the patient, which was absent in DNA of the parents. The mutation was not reported previously and was not found in the 478 healthy controls. We obtained about 8000-fold coverage from high-depth sequencing of DNA from peripheral blood of the patient and the parents. The disease-associated allele C identified in the patient was not observed in the parents. The number of reads with G allele (3984 reads) was roughly equal to that of C allele (4019 reads) from the targeted reanalysis of DNA of the patient. The results from high-depth sequencing indicated a de novo mutation in maternal germ cells. Conclusions We identified a new GNAS gene mutation (c.479G>C, p.R160P) caused PHP-Ia in a patient. Our results suggested the mutation was a maternal germline de novo mutation.