In water conservancy and hydropower engineering, prestressed anchor cables (or bars) are commonly used to reinforce unstable slopes and improve their stability. However, the design of prestressed anchorage has so far been mostly over-conservative, resulting in excessive waste. The anchorage design theory and method need further study. Through analysis of factors affecting rock-anchor design parameters and by applying optimization and decision theory, this paper proposes a method for determining rock-anchor design parameters and establishes the rock-anchor support design decision-support system RSASYS, providing designers with a new analysis approach. Practice proves the method feasible and of great economic benefit.