The invention relates to an automatic micro imaging instrument used for detecting cast-off cells and a detecting method, in particular to an automatic analysis instrument of a uterine neck cell smear and a detecting method. The instrument comprises a microscope, a three dimensional automatic object stage, an automatic film loading device and a control system thereof, a light system, a stable system, a light filter, a computer, a display and a digital camcorder, etc. The detecting method comprises the following steps: the smear is analyzed and colored, then arranged on the automatic object stage which moves up and down to focus automatically, when the automatic object stage moves horizontally, the smear scanning is realized; cell images are amplified by the microscope, picked up and sent to the computer to carry out image processing and analyzing. The detected cells are filtered by two-value filtration to remove the noises, the area and the perimeter are measured, and the impurity is deleted, then the spectrum and other shape parameters of the cells are measured; the measured cells are classified with a pattern recognition algorithm into the cast-off cells, white blood cells, lymphocyte cells and the impurity; after a viewing field is finished, the cells move down automatically, auxiliary diagnostic data is obtained by the computer. The instrument and the detecting method have the advantages of high degree of automation, accurate analysis results and reliability.