The research on fossils can help scientists understand the biological evolution and judge the stratigraphic age. The change of geological elements in different ages is a popular topic in geological research. In order to study the changes of geological elements in different ages, we used Micro LIBS to study the element distribution in the ammonite stones. The asymmetric least squares method is used to remove the baseline of the spectral data, and we determine the optimal fitting parameters. The average normalization algorithm is used to reduce the relative standard deviation, and the multiple linear regression algorithm is used to calculate the regression equation of this model. First, the optimal experimental parameters were determined by preliminary experiments: the wavelength of laser is 1 064 nm, the frequency of laser pulse is 30 Hz, and the acquisition delay is 700 ns. Secondly, 12 pieces of rocks whose contents were already known were selected, 9 samples were randomly extracted for testing, and the remaining 3 samples were for predicting. Ca Ⅱ 393186 nm, Ca Ⅰ 422856 nm, Ca Ⅰ 445572 nm, Ca Ⅱ 559031 nm and Ca Ⅰ 61661 nm were selected to establish the quantitative analysis model of Ca element with a prediction accuracy of 929%. Then, a 5×5 area was scanned to get a series of atomic spectrum data. According to the quantitative analysis model of Ca element, the lateral distribution map of Ca element can be got, and its horizontal resolution is better than 100 μm. Finally, the 6th, 11th, and 16th spectra data of each test point were selected for processing to get a lateral distribution map of Ca elements. The longitudinal distribution of Ca element in ammonite stones can be got by comparison. The Ammonite can not only be used as evidence to judge the age of the bottom layer, but also the elemental information of the bottom layer of the fossil can be inferred by studying the element distribution and content of Ammonite. This research has guiding significance for the evolution of the geology of shallow sea stratum and environmental changes.