The paper discusses some fundamental theoretical problems,such as features of the concepts of attached obligation,function and shape,law characteristics and system value,and does some comprisons of the silent obligations between Britain and America.

In this paper,the necessity of the uses of implied terms system of contract in domestic construction contract is pointed out,the main contents of the implied terms of contract about contractor s key obligations in construction contract are analyzed and studied.

The former,which cann t become contractual obligation, is additional article of validity of a contract.

A deep exploration of the basis of its occurrence, its relationship with contractual obligations and paying obligations, its connotation, and its value target is worthwhile.

Quite going beyond the traditional sense of contractual obligation, modern contract law protects not only the contract itself, but also the entire course of transaction.