【目的】 法氏柴胡宽蛾Depressaria falkovitshi Lvovsky为三岛柴胡的一种优势害虫，主要以幼虫危害柴胡的叶片、茎尖、花序，幼虫龄期的确定可以为其生物学特性的研究和防治措施的制定与实施提供理论依据。【方法】 本文通过测量法氏柴胡宽蛾幼虫头壳宽度和体长，根据所测数据的频次分布及Dyar定律推测出法氏柴胡宽蛾幼虫的虫龄数。运用Crosby生长法则和线性回归分析验证分龄的合理性。【结果】 研究表明，法氏柴胡宽蛾的幼虫期共分为5龄，1~5龄幼虫的头壳宽度（mm）分别为0.163±0.006，0.262±0.012，0.437±0.025，0.690±0.038，1.116±0.051。幼虫头壳宽度（y）与虫龄（x）的回归方程为y = 0.101e^0.4815x，R² = 0.9998。【结论】 头壳宽度适宜于法氏柴胡宽蛾幼虫期的分龄，而体长由于变异大不宜用作幼虫龄期的划分。

 [Objectives]  The larvae of Depressaria falkovitshi Lvovsky (Lepidoptera: Elachistidae: Depressariinae) are an important pest of Bupleurum falcatum L.; damaging the leaves, stem tips and inflorescences of this plant. Classification of D. falkovitshi larval instars could be important for developing control measures for this pest. [Methods]  In this study, the head capsule width and body length of larvae were measured ,then the larval instars were identified on the basis of frequency histograms and Dyar’s law. The rationality of instars was verified by Crosby’s growth rule and linear regression. [Results]  The results suggest that the larval stage of D. falkovitshi is comprised of 5 instars with an average head capsule width (mm) of; 0.163±0.006, 0.262±0.012, 0.437±0.025, 0.690±0.038 and 1.116±0.051, respectively. The linear relationship between head capsule width (y) and instar (x) was: y=0.101e^0.4815x, R² = 0.9998. [Conclusion]  Head capsule width can be used  to reliably identify larval instars but body length is too variable.