The invention relates to a fermented alga liquid fertilizer synergist and a production method and an application thereof. The fertilizer synergist is a fermented alga liquid fertilizer synergist obtained by performing microbial fermentation on natural alga serving as a raw material, preparing an alga extracting solution through procedures such as alkali extraction, filter pressing and the like, and adding chelated state trace elements into a liquid. Due to the adoption of the fermented alga liquid fertilizer synergist disclosed by the invention, the soil urease activity can be suppressed remarkably, slow release of a fertilizer is enhanced, and the crop yield can be increased remarkably; trace elements such as boron, zinc, manganese and the like which are generally lacked in soil in China can be replenished; root growth of crops can be facilitated, root activity is increased, and the capabilities of crops of absorbing phosphorus, potassium and medium and trace elements are increased; and the fermented alga liquid fertilizer synergist has the characteristics of reduction in nutrient loss, avoidance of secondary pollution, reduction in the production cost of a synergic fertilizer, wide raw material source, convenience for adding in fertilizer production and the like, and has a wide application prospect in the production of the synergic fertilizer.