The purpose of this study is to evaluate the efficacy and safety of F520 combined with paclitaxel plus carboplatin as first-line treatment for cancer of unknown primary.This study consists of two parts, with a total planned enrollment of 402 patients.Primary Endpoint is Objective Response Rate (ORR) assessed by Independent Review Committee (IRC) based on RECIST 1.1 Secondary Endpoints included ORR、PFS、DOR；Incidence and severity of Adverse Events (AE), Serious Adverse Events (SAE), and abnormal laboratory parameters.