With the invasion of alien organisms becoming a global environmental problem, it needs a instrument for the on-line detection. A portable image analyzer which can realize on-line larva detection for plant quarantine is demostrated. The instrument adopts zooming optical system with a 0.7～9 zoom by rotating zoom ring. The apparatus has a light emitting diode(LED) reflective uniform illumination system optimized by using orthogonal experiment method. In the scope of Φ50 mm, the uniformity of illumination can reach up to 83.7%. A larva-recognition software is designed to identify and analyze the larvae. The proposed portable larvae image analyzer provides a new on-line detection method.