An appropriate and balanced coverage of mathematical content is of ultimate importance for quality early mathematics education. This study developed and validated the Early Mathematical Content Inventory (EMCI) for preschool teachers for achieving a balanced coverage of mathematics content in early mathematics education. The inventory was validated with 6,356 preschool teachers in China and included 17 items, which can be factored into three components: i.e., pre-numeracy, numeracy and shape, and complex mathematics activities. The results showed that numeracy and shape were the most common concepts covered in mathematical activities in Chinese preschools, followed by pre-numeracy and finally complex mathematics activities. The age group taught and the number of years of preschool teaching significantly predicted the types of mathematical content covered. Our findings suggest preschool teachers adopt the EMCI in teaching practices. More support for teachers’ understanding of what content to be covered in early mathematics education should be provided in both teacher preparation and professional development programs.