In light extinction particle sizing technique a first kind of Fredholm integral equation should be solved. How to determine the upper and lower limits of the integral equation or the measurable range during the measurement is am an existing problem. This paper pints a new algorithm-a self adaptive algorithm of dependent mode, which will dermine the upper and lower limits of the integral equation automatically in terms of the value of k, one distribution parameter of the two parameter's distribution function. Numerical simulation and experiments were carried out to confirm this algorithm.