Lactic acid bacteria (LAB) are promising candidates for live bacterial drug delivery systems owing to their safety, probiotic properties, and ability to colonize the intestinal tract. At present, most research focuses on engineering LAB as carriers for the delivery of therapeutic proteins. As model organisms equipped with a versatile set of genetic tools, LAB can be readily modified to target various diseases and yield significant therapeutic effects. LAB-based carriers offer multiple advantages, including non-invasive delivery, ease of genetic manipulation, and suitability for large-scale production. Consequently, the use of LAB as recombinant protein expression vectors has attracted extensive research interest worldwide. The foundational principles, strategies for enhancing bioavailability, genetic engineering approaches, and the current research and application status of LAB-based drug delivery systems were summarized in this paper.