Abstract: In recent years, legal experts' opinion papers have been increasingly growing, and have had certain effect on the judicial trial. However, they are neither the testimony of witnesses nor conclusions, and are also different from the defence and the agent word. It lies inside the area of theoretical explanation. It will help protect the legitimate interests of the "vulnerable", and contributes to the justice and democracy. From the comparative law perspective, taking example by the "amicus curiae" system in the United States, we need to give it the right position in legislation, clear the subject qualification, and specify the main body to invite, burden of expense, and the interests of the parties. The legal experts could only comment on the application of law, but not the legal facts.