A knife structure allowing rapid and safe replacement, and secure positioning of, a blade disposed along with its handle structure; wherein the knife structure includes the blade, an engaging and pushing plate, and an engaging and positioning base; a first connecting member is fixed to the engaging and pushing plate and is movably engaged in the blade and the engaging and positioning base; an engaging block is fixed to the engaging and positioning base and is movably engaged in the blade and the engaging and pushing plate; and the first connecting member is locked to the engaging and positioning base upon assembly of the knife structure. Once unlocked via a pushing and unlocking key, the first connecting member can be used to push the engaging and pushing plate and the blade simultaneously, allowing the blade to be pushed out of engagement with the engaging block and then detached for replacement.