The invention discloses compound clarifying agent and a preparation method thereof. The compound clarifying agent is prepared by compounding 10 parts of cerium oxide, 20 parts of antimony oxide, 3 parts of lepidolite and 60 parts of calcium sulfate. The compound clarifying agent is compounded by the cerium oxide, the antimony oxide, the lepidolite and the calcium sulfate, overcomes the difficulty of high cost caused by adopting single cerium oxide, antimony oxide and lepidolite as the clarifying agent, and has low poison and better clarifying effect; the invention further discloses a method for preparing E glass fiber by adopting the compound clarifying agent; and the method uses the gradual clarifying theory of the compound clarifying agent to gradually decompose and clarify when glass is molten, so that the clarifying capacity is always in a vigorous state, and the E glass fiber with few impurities and good quality is obtained.