We report the evaluation of the systematic shifts and the total uncertainty of an 27 Al + optical clock sympathetically cooled by a 40 Ca + ion. Due to its relatively low Doppler cooling limit and the suitability of its mass ratio to the 27 Al + ion, the time dilation shift of the secular motion of our 40 Ca + − 27 Al + clock is smaller than the shift of a 25 Mg + − 27 Al + clock. The total systematic uncertainty of our 40 Ca + − 27 Al + quantum logic clock has been estimated to be 7.6 × 10 −18 , which was mainly limited by the uncertainty of the quadratic Zeeman shift.