The invention relates to the field of algae inhibitors, in particular to a carotane sesquiterpene compound of an alga growth fungus source as well as a preparation and application thereof to inhibition of microalgae. A specific structural formula is shown in (I) shown in the description. The preparation method comprises the following steps: inoculating alga growth fungus trichoderma virens Y13-3 into a fungus medium for performing fermentation culture, separating and purifying the fermentation product, thereby obtaining the carotane sesquiterpene compound shown as the formula (I). According tothe carotane sesquiterpene compound prepared in the invention, microalgae activity inhibition experiments prove that a median inhibitory concentration of the compound on the microalgae can reach 0.14microgram per milliliter.