Insulin-like growth factor (IGF) signaling pathway plays an important regulatory role in the development, reproduction and growth of the organism. IGF acid-labile subunit (IGFALS) is an important member of the IGF system. After forming a trimer with IGF and IGFBP, IGFALS can extend the half-life of IGF and thus participate in the regulation of IGF system. In this study, two IGFALS genes were firstly screened and identified from the razor clam Sinonovacula constricta transcriptome, including complete ORF, named IGFALSa and IGFALSb. The length of IGFALSa cDNA sequence is 4 809 bp, encoding 996 amino acids; The length of cDNA sequence of IGFALSb gene is 1 925 bp, encoding 546 amino acids. During the developmental periods, IGFALSa and IGFALSb were only expressed in a small amount in the early developmental stages, and their expression gradually increased with the larval development. In different tissues, IGFALSa was highly expressed, specifically in mantle and gonads. IGFALSb is highly expressed, specifically in siphon. Short-term stimulation to juveniles was performed with different mass concentrations of insulin. When the mass concentration of insulin was 10 mg/L, both IGFALS genes were significantly up-regulated, and the expressions of the downstream genes IRS, PDK1, and MPKP1 were also significantly up-regulated. Further feeding the juvenile with 10 mg/L of insulin for 40 days, the daily growth rates of shell length and shell width were significantly improved.