The novel Half a Lifelong Romance contains a large number of idioms, which can be classified into three categories: four-character idioms, idiomatic expressions and proverbs, fully displaying the unique characteristics of Chang’s language style. Based on the truth-seeking-utility-attaining continuum evaluation model, this paper evaluates the idiom translation in Karen’s English version from the perspective of translator behavior criticism. It finds that in the truth-seeking-utility-attaining continuum, Karen’s translation slides towards the utility end. In other words, she follows the principle of “prioritizing utility-attaining while balancing truth-seeking” in her translation so as to serve readers and communities. Such a principle develops from her personal background as well as her aim and pursuit in translating. Meanwhile, her translation practice of idioms inspires us to flexibly select and employ the translation strategies to achieve equilibrium in truth-seeking and utility-attaining.