Why is 系 pronounced xì (not jì) in the idiom 解铃还须系铃人?
You should know that, the pronunciation of 解铃还须系铃人 is a long lasting debate. If you want to take an exam in mainland China, you must remember the most updated pronunciation in the dictionary. Otherwise, you're free to choose the one you feel the most feasible. The closest meaning of xi and ji as a verb are shown below. Quoted 百度百科.

&quot;系（jì）&quot;是单义词，只有一个义项：打结、扣。运用范围很小，只用于“系鞋带”、“系围裙”、“系领带”、“系扣子”、系红领巾等词语中。


“系（xì）”是多义词，在《现代汉语词典》上有8个义项：
...
(7) 拴、绑、束缚：系马、系缚、不系之舟
...