<p>Private Set Intersection (PSI) enables two mutually distrusting parties to calculate the intersection of two private sets without disclosing any additional elements. Updatable PSI not only allows parties to update elements in the original set but also avoids redundant computations of non-updated elements from scratch. This is particularly useful in scenarios where element updates occur periodically and frequently, like contact tracing. In such cases, revealing the cardinality of the intersection is frequently adequate. However, existing PSI cardinality (PSI-CA) protocols do not effectively address this problem as they primarily focus on static sets without considering element updates. To tackle this concern, a cloud-assisted updatable PSI-CA protocol is presented. Our protocol enables participants to calculate the intersection cardinality by leveraging cloud servers while also supporting the addition and removal of elements from the original set. What’s more, this proposed protocol can be completed in constant-round interactions. Its security proof and performance analysis also highlight its security and effectiveness.</p>