In pig production, food conversion ratio and profit can be evaluated by real time detection of pig live weight. Traditional pig weight detections usually require direct contact with pigs, which are limited by its low efficiency and result in a lot of stresses even to death. The non-contact detection of pig body weight has become a challenge in pig production for decades. Digital image analysis and machine vision method enable the real time estimation of pig live weight by detecting pig critical body dimensions without any contact. This article elucidated the advantages and limitations of each detection method of pig body weight by comparing the system framework and estimation models. The research trends of contactless pig weight estimation were analyzed as well.