Background, aim, and scope Global warming caused by emission of greenhouse gases dominated by CO_2 has become a severe challenge to human being. Many measures have been taken to combat global warming all over the world. What is worth mentioning is the CO_2 geological sequestration, it is an effective and burgeoning measure to achieve carbon emission reduction. The CO_2 sequestration potential is very huge in Ordos Basin. So, in this paper the formation water characteristics in Ma5_1 submember of Majiagou formation are systematically studied, which is the precondition for further engineering design of CO_2 sequestration in Ordos Basin. Materials and methods Based on the direct observations and experimental tests of deep drilling samplings, the method of single index classification evaluation and multiple index comprehensive evaluation is adopted. The physical properties, chemical properties, formation water types and hydrochemical characteristic parameters of the Majiagou formation Ma5_1 submember are analyzed in detail. Results The results indicate that the pH of the water ranges from 3.5 to 8.6, mainly from 4.5 to 6.5; the water density ranges from 1.0 g · cm~(-3) to 1.5 g · cm~(-3), mostly 1.1 1.3 g · cm~(-3); the mineralization degree is generally greater than 50 g · L~(-1), and average mineralization degree is 166 g · L~(-1); the highest concentration ion is Cl~-, followed by Ca~(2+); the concentration order of the ions is r Cl~-> r Ca~(2+)>r ( Na~+ + K~+ ) >rHCO_3-Mg~(2+)>rSO_4~(2-)>r , and the formation water is CaCl_2 type. The analysis results of water chemical characteristic parameters show that Na / Cl factor (r Na~+ / r Cl~-) ranges from 0.18 to 0.55, 0.34 on average; Cl / Mg factor (r Cl~-/ r Mg~(2+)) ranges from 11.04 to 165.16, 32.11 on average; desulfurization factor (100 * rSO_4~(2-)/ r Cl~-) ranges from 0 to 0.45, 0.09 on average; Mg / Ca factor (r Mg~(2+) / r Ca~(2+)) range from 0.01 to 0.30, 0.17 on average. Discussion Most samples have a pH of 4.5 6.5, which does not meet the drinking water standard (6.5 8.5). The density is generally 1.1 1.3 g · cm~(-3), indicating that the salinity may be high.