The invention relates to a novel environment-friendly tree trunk whitewashing agent and a preparation method thereof, belongs to the technical field of tree maintenance, and provides the novel environment-friendly tree trunk whitewashing agent in order to solve the problems that a traditional whitewashing agent is poor in whitewashing effect, trunk-boring pests are difficult to control, the trunk-boring pests are prone to falling off, the heat preservation effect is poor, and the trunk-boring pests have toxicity and are harmful to the environment. The whitening agent comprises the following raw materials: alginate, calcium sulfate, diatomite, beta-cyclodextrine, white wood fiber, instant silicate, a retarder, xanthan gum and a bifenthrin aqueous solution, and the whitening agent is neutral, is friendly to the ecological environment, does not cause soil hardening and salinization, has a gel property, can fill bark cracks of tree trunks, and has a good whitening effect. The tree trunk protection agent has the advantages that the tree trunk protection agent can well contact with overwintering eggs in tree bark cracks, the killing effect is improved, the tree trunk protection agent has certain thickness and hardness after being coated on tree trunks, damage of trunk borers to the tree trunks can be reduced, and the product is mainly used for protecting the trees.