Novice programmers are difficult to debug on their own because of their lacking of prior knowledge. If we want to help them, first we need to able to check the correctness of a novice¡¦s program. And whenever any error is found, we could provide some suggestion to assist them in debugging. We use concolic testing algorithm to automatically generate test inputs. The test inputs generation of the concolic testing is directed by negating path conditions and is produced by solving path constraints. By using of concolic testing, we are able to explore as much more branches as we can. And once we found an error, we will try to locate it for novice programmers. We propose a new method called concolic debugging. Its idea comes from concolic testing. The concolic debugging algorithm initiates with a given failed test, and try to locate the faulty block by negating and backtracking the path conditions of the failed test. We use concolic testing to improve assessing style of the automated assessment system. 86.67% of our sample programs are successfully assessed by concolic testing algorithm on our new automated assessment system. And we also found our concolic debugging is much more stable and accuracy on fault localization then spectrum-based fault localization.