It is reported in this paper that calcium can be determined with MTB as chromogenic reagent by isoabsorption dual-wavelength elimination. The measuring wavelength and referential wavelength were 613.7 and 590.0nm, respectively. No masking reagent was necessary in the analysis. The average recovery and RSD of calcium are 98.98% and 0.81%, respectively. Compared with titration, this method is rapid, simple and precise.