Lehre vom fakultatives Kollisionsrecht, put forward by A. Flessner and given a detailed description by Th. M. De Boer, regards that paying attention to procedural interest of the parties should be one of the yardsticks to affirm the applicable law, even applying lex fori. This theory seems to contradict the most significant relationship principle and the lex fori principle of traditional international private law theory. In fact, they can blend well and complement each other, and can provide theoretical support for improving and enriching China's foreign civil and commercial judicial practice.