Magnesium (Mg) and calcium (Ca) are essential for numerous kinds of metabolisms in human body. To investigate the associations between Mg and Ca and the ratio of Ca to Mg (Ca/Mg) in whole blood with metabolic syndrome (MetS) in a Chinese population, a matched case-control study including 204 MetS patients and 204 healthy controls (aged 48-89) was carried out in 2011. MetS were diagnosed according to the criteria of Chinese Diabetes Society. Controls had no abnormal metabolic components and were matched with cases by age, gender and region. Blood samples were collected in the morning after an overnight fast. Whole blood Mg and Ca were determined by flame atomic absorption spectrometry. Subjects who were male constituted 44.1% of the part of this study. The average age was 64.0 ± 7.18, and the average body mass index was 24.3 ± 3.75. The MetS group showed significantly higher Mg and lower Ca and Ca/Mg as compared with the control group. Comparing with the bottom tertile (T1) of Mg, increased ORs for MetS were found in median tertile (T2) and top tertile (T3) of Mg. For Ca, T2 and T3 were negatively associated with MetS. Inverse relationship was also found between Ca/Mg ratio and MetS. Our findings suggested that increased Mg and decreased Ca and Ca/Mg in whole blood were correlated with MetS in Chinese adults.