Disclosed is the use of a radiolabeled anti-nanobody in the prognosis and diagnosis of cancers. In particular, disclosed is an immunoconjugate for detecting a PD-L1 molecule. The immunoconjugate comprises the VHH chain of a specific anti-PD-L1 nanobody and a radionuclide, and can be used for non-invasive detecting of expression of the object PD-L1 to be detected. The immunoconjugate of the invention has small size and high specificity, and is suitable for systemic detection which simultaneously targets primary and metastatic tumors, and has high accuracy and low radiation dose.