As a typical disk galaxy, our Milky Way is a unique laboratory for studying the matter constitution and distribution and testing the formation and evolution scenarios of galaxies in general. Although the study of the Milky Way has a long history, many fundamental questions still remain to be answered. In the past decades, extensive large surveys have been carried out to obtain multi-dimensional astrophysical information (such as 3D position, 3D motion, chemical composition etc.) for millions to billions of stars. Many more surveys and data products will be continually produced in the coming decades. These blowout growth of stellar data provide unprecedented opportunities for deep understanding and new discoveries of our Milky Way, but at the same time also propose huge challenges for data analysis and explanation. In this talk, I will have a brief summary on our past efforts toward a comprehensive census of stellar populations of the Milky Way, particularly for the disk, with the LAMOST low-resolution spectroscopic survey. Stellar spectroscopy can reveal a wealth of astrophysical information of stars, thus is an essential way of studying our Galaxy. I will first present our extensive efforts on delivering stellar astrophysical labels from the LAMOST R~1800 spectra for millions of stars, and then introduce what we have learned about the stellar population and assemblage history of the Milky Way disk from this large data set.