The invention discloses a botanical antiviral agent with a Rhodiola eurycarpa extract as an effective component and its preparation method. The preparation can be in the form of a soluble liquid agent, a water agent and a microemulsion. The preparation contains 10%-40% of the Rhodiola eurycarpa extract and the balance assistants. The anti-plant virus agent can be used for preventing and treating a cucumber mosaic virus (CMV) disease on tobaccos, chillies, tomatoes, cucumbers and other crops. In addition to good antiviral effects, the anti-plant virus agent is also safe to the environment, human, livestock, natural enemies of pests and other beneficial organisms. The anti-plant virus agent has a simple preparation process and a low cost, thus being suitable for popularization and application.