"合约式联营企业" can be translated into English as "contractual joint venture enterprise" or simply "contractual joint venture".
"Contractual joint venture" is a commonly used term in business and legal contexts to refer to a cooperative arrangement between two or more parties based on a contract, without necessarily forming a new legal entity (though in some cases, a separate legal entity may be established under the contract).
Adding "enterprise" at the end (i.e., "contractual joint venture enterprise") emphasizes that it is a business undertaking or a commercial entity formed through such a contractual arrangement.
In many cases, especially in international business or legal documents, "contractual joint venture" is sufficient and widely understood. However, if you want to be more explicit about it being a business entity, "contractual joint venture enterprise" is also acceptable.