In this paper, CuInZnxS2+x (CIZS, x = 1) have been used as cadmium-free core materials to synthesize CIZS-based core/shell quantum dots (QDs). By introducing extra Zn precursor to CIZS cores, two-time separate shell growth process, and continuous long time replenishment of shell precursor, cation exchange between Cu and Zn ions during the growth of the ZnS shell has been greatly reduced and as large as 7.8 +/- 1.2 nm CIZS/ZnS//ZnS with an average of 3.1 nm shell were synthesized successfully. Such thick shell CIZS/ZnS//ZnS core/shell QDs had high stability and high quantum yields (77%). Compared with the thin shell CIZS/ZnS QDs, the thick shell CIZS/ZnS//ZnS QDs still showed excellent PLQY (up to 58%) and stability in different environment after transfer to aqueous solution. We adopted the thick shell cadmium-free QDs as fluorescence label into lateral flow immunoassay for quantitative detection of C-reactive protein, and the limit of detection was 5.8 ng/mL, which was almost as low as that of cadmium-based quantum dot lateral flow test strip. Therefore, this kind of thick shell CIZS/ZnS//ZnS QDs can be used as probes for ultra-sensitive detection, and the further development of this kind of QDs has immense potential for future convenient and cost-effective in vitro cadmium-free nano-medical diagnostic kits.