According to the theory of global administrative law, accountability, transparency, public participation, justification and review procedure are indispensable elements of investment treaty arbitration. As a main way of public participation, amicus curiae, however, has limited influence on existing practices. Whether the applicant is qualified, whether he or she can participate and express, and whether the expression can be accepted completely are entirely at the discretion of the arbitral tribunals. In order to improve the legitimacy of investment treaty arbitration, the rules and procedure of amicus curiae need to be modified under the theory of global administrative law. In combination with the arbitration rules and the relevant rules of the latest investment agreement, to improve the amicus curiae rules, we should clarify the main review standards of amicus curiae and reduce the restrictions on the extensive participation of amicus curiae by secondary standards such as identity, major interest relationship and procedural burden; we should formulate an effective feedback template, i.e. introduction, information disclosure, observations of the complainant and the respondent, and conclusion; and we should use procedural control tools by limiting length, allowing joint statements, and setting the time limit, etc.