A lens set that can achieve large-area uniform light spot based on LED extended light source is designed by means of freeform surface method in combination with Snell's law and energy conservation. The lens set consists of a primary lens and a secondary lens. The primary lens regulates energy to achieve uniformity adjustment, while the secondary lens shrinks the LED luminous angle to reduce the lens set structure. The Monte Carlo ray-tracing simulation shows that a single lens set can achieve a square uniform spot on the receiving surface, with an effective light flux ratio of 95.5% and a uniformity illumination of 95.7%, indicating that the lens set equipped with LED source has excellent effective light flux ratio and illumination uniformity. While for lens array, simulation shows that it can also achieve a square uniform spot on the receiving surface, with an effective light flux ratio and uniformity greater than 95%, indicating that the lens set can be extended to the lens array adapted to planner multi-LED extended source. The results show that the designed lens set and lens array have promising potential applications in satisfying high-power light sources of the fluorescence-based inverse measurement devices.