An important trend in embedded system is integrating functions with different level of importance into a sharing hardware platform, which is called mixed-criticality system. Most of the existing mixed-criticality theory did not support switching the system criticality from high to low in order to guarantee the jobs with higher criticality, which is not good for the overall performance of the system. To deal with this problem, this paper expands the traditional demand boundary analysis theory to the mixed-criticality systems, presenting the concept of dynamical demand boundary for mixed-criticality jobs, which represents the dynamical demand of jobs in run-time as a vector. And then, based on the concept of slack time for mixed-criticality jobs and the criticality of system, the paper presents an algorithm CSDDB (criticality switch based on dynamical demand boundary). The algorithm chooses the criticality with the minimum slack time as the execution criticality of the system to take full advantage of system resources and to guarantee the execution of jobs with lower criticality without affecting the schedulability of high criticality jobs. Experiments with randomly generated workload show that CSDDB makes more than 10% of progress in guaranteeing the system criticality and the completion of jobs set compared with the existing research.