Control of administrative discretion is an important concern in the development of the rule of law. According to Wade and Forsyth, the rule of law has four meanings, and one of them is that “government should be conducted within a framework of recognized rules and principles which restrict discretionary power”. While judicial review is crucial to discerning and enforcing the limits of discretionary power, its interaction with the constitutional principle of separation of powers invites careful consideration of its basis. In reconciling the two principles, British courts have developed reasonableness as the primary basis for judicial review, which was further impacted by continental proportionality after joining the EU. On the other hand, China has adopted distinct criteria for judicial review, namely “manifest unfairness” and “manifest unjustness”. This paper compares judicial control of administrative discretion in the UK and China and argues that despite the gap, China still has a tendency toward judicial initiative, which entails the “thick” meaning of the rule of law. Further, this paper proposes that China should bridge the gap in a progressive way. Part I explores the principle of reasonableness and the principle of proportionality in the UK and reflects on the developmental history of the British approach. Part II examines the situation in China and provides a comparative analysis with the UK’s approach. Part III puts forward suggestions.