The characters literally combine to mean: “Once (一) it is established (成), it does not (不) change (变).” This literal meaning is very close to its idiomatic meaning, making it relatively easy for learners to remember.
While Chinese culture values stability and order, 一成不变 (yī chéng bù biàn) highlights the deep-seated cultural understanding that an inability to adapt is a critical weakness. This concept is a sharp criticism in a society that, from ancient philosophy (like the I Ching or Book of Changes) to modern business, prizes adaptability and resourcefulness (随机应变, suí jī yìng biàn). A good Western comparison is the concept of “being stuck in one's ways,” but 一成不变 is more versatile. You can apply it not just to a person's habits but also to company policies, social rules, or even a boring lifestyle. It's a stronger and more definitive criticism than simply calling something “traditional.” While a tradition (传统, chuántǒng) can be a source of pride, something that is 一成不变 is seen as stagnant and in need of reform. It implies a foolish resistance to the natural, necessary flow of progress and change.
This idiom is common in both spoken and written Chinese. It's a powerful way to express frustration or criticism.