The advocacy of the construction of “smart court” and the need of prevention and control of the Covid-19 epidemic situation have promoted the rapid development of the court's online trial of criminal cases. However, there are some problems in online trial, such as impairing the principle of direct speech, undermining the dignity of justice, and insufficient protection of the defendant's right to defense. It not only aggravates the formalization of the trial, but also leads to the reduction of the fairness and judicial authority of the trial procedure. The online trial of criminal cases is only an exception and supplement to the offline physical space trial. The online trial is mainly applicable to the cases in the quick trial procedure and agreed by the defendant, the pre-trial meeting and the testimony of witnesses and experts under special circumstances, and cannot be normalized and generalized. The court should respect and protect the defendant's choice of procedure. Legal aid should cover all cases that should be tried online. The Supreme Court should formulate technical specifications for online trial and apply 5G technology in online trial as soon as possible.