A neuron is the basic unit of the structure and function of the biological nervous system. The detection of neuronal firing is developed rapidly in recent years. However, the cell activity may be affected by most of the technologies. Considering that the concentration of calcium ion in the cell body plays a key role in modulating neuronal firing activity and the solution refractive index is closely related to its concentration, the optical phase information of a neuron is employed in this paper to describe its firing rhythm with a label-free approach. Based on the associated optical parameters, a neuron model with soma and synapses is presented. Due to the phase information of a sample is actually a set of spatial distributed data, the edge detection is performed with a gradient operator to determine the sampling point of phase data for dynamic monitoring. Since the calcium in neurons mainly exist in neuron cytoplasm, Cacl<sub>2</sub> solution is used to simulate the liquid environment in neuron cytoplasm, and the relationship between the refractive index of the solution and the change of the ion concentration is measured through experiments. To demonstrate the phase variation during neuron firing, we take periodic change in calcium concentration in the neuron plasma as an example by simulation. According to the above result, the similar periodic variation of refractive index is be obtained by numerical calculation, as well as that of phase value at the sampling point. From the phase distribution, the variation of phase value can be observed clearly. This work could provide the basis for the label-free detection and characterization of neuron firing.