This paper presents WASIT, a powerful specification-driven differential testing framework for WebAssembly (Wasm) system interface (WASI) implementations. WASIT invents several innovative techniques to address the challenges facing state-of-the-art testing approaches when applied to WASI implementations. Specifically, it introduces real-time resource abstraction and tracking to facilitate the generation of meaningful and dependent WASI function calls. It also creates a domain-specific language to automatically filter out uninteresting WASI function argument values by augmenting the WASI specification. Finally, it adopts a decoupled system architecture to achieve smooth co-evolution with WASI. Our evaluation shows that WASIT successfully found 48 new WASI-specific bugs in six popular Wasm runtimes, with 41 confirmed, 37 fixed, and three CVEs assigned.