K-space is the basis of signal spatially encoding in magnetic resonance imaging. Accurately understanding signal expression of K-space is the key step to understand the principles behind magnetic resonance imaging. In this work, we take conventional spin echo pulse sequence as an example to comprehensively derive the signal expression, by combining the signal spatial encoding process. In the signal acquisition step, a physical magnetization model was used to simplify the signal expression under the Faraday^,s law of electromagnetic induction. In the mathematically processing step, two-dimensional integral expression of K-space data is derived based on the micro-element analysis. The Details and the corresponding theory in each derivation step are clearly demonstrated. The model connects the key concepts and theories of magnetic resonance imaging, which can help to systematically understand the K-space concepts and the principles behind magnetic resonance imaging.