As single sided ultrasonic algorithm introduced by code CECS 21:2000 is not accurate in practical application in assessment of the damaged depth of concrete after fire, improvement has been made in this paper. Hyperbola curves were used to simulate the trends of damage along concrete depth while parabolic curves were used to simulate the traces of ultrasonic wave in different concrete depth. The programming and calculation work were completed by Matlab compiler. Calculating results from modified algorithm shows more accuracy by comparing it with measured data. Modified algorithm can assess the damaged depth of concrete more feasible and accurate.