The present invention discloses a device for detecting blood cells and a microscopic hole sensor assembly thereof. The device includes a microscopic hole sensor assembly positioned between a front pool and a back pool. The microscopic hole sensor assembly further includes a microscopic hole plate with a microscopic hole positioned thereon for communicating the front pool and the back pool. The end surface of the inlet is a flow-guiding surface gradually narrowing from the front pool to the back pool. The end surface of the outlet is a downstream surface gradually distending and extending towards the back pool. A straight effective orifice area is formed connecting the ends of the flow-guiding surface and the downstream surface.