A new active atmospheric detection means of the imaging lidar is used in the horizontal atmospheric transmittance detection. A matrix image dislocation addition method is proposed to improve the signal to noise ratio. When the signal which is changing with the distance is obtained, an inversion method combined with the slope method and Klett integration is adopted to retrieve the extinction coefficient distribution in 1 km horizontal transmission path. The horizontal atmospheric transmittance can be achieved based on the path integral of extinction coefficient. By comparing the inversion results of comparing experiments with the atmospheric transmittance obtained in direct measurements, we find that the correlation coefficient is over 0.7 and the relative error is within 20%. The imaging lidar can be an instrument for the study on detecting the horizontal atmospheric transmittance in the finite range.