The fractal component in the daytime healthy heartbeat interval data is studied from the perspective of cascade in fluid turbulence. Based on the electrophysiology of the heart muscle cell, a bounded random cascade model is assumed and the scaling property of the model is derived. In application, a "cascade decomposition" is proposed to extract the model parameter based on the experimental data. Simulation results show that healthy heart rate variability (HRV) can be well captured by the multiplicative process and imply the significance of sympatho-vagal interaction in the fractal component of long-term healthy HRV.