Abstract: The reform of the litigation system which centers on trial is the direction of criminal justice reform in China,and the essential component of the reform of litigation system is to regard trial as the center. The "Criminal Procedure Law" has made corresponding provisions on the system of testifying on key witnesses,but the provisions on defendants' right of inquiry have to be further elaborated. Clause 3 of Article 6(d)European Convention on Human Rights and the sixth amendment of United State Constitution are set to protect the defendant's right of inquiry,and both of them do have the precedent of guiding significance. The European Court of Human Rights has experienced a change from "the only and decisive rule" to the "three-step test rule";the guarantee of America's right to inquiry also has undergone the transition from "reliability standard" to "testimony standard",which plays an important role in safeguarding the defendant's right. The guarantee of China's right of inquiry should be specified on the basis of the existing key witness system. The principle of the witness is required to testify in court,with an exception of testifying out of court,should be upheld, and exceptions and relief measures should be clearly stipulated to protect the right of inquiry from time and space.