In the era of 5G and beyond, different service scenarios have put forward rich and differentiated requirements for the carrier network. The emergence of flexible Ethernet technology has met the needs of high-speed transmission and flexible bandwidth configuration. However, the current FlexE transmission mechanism based on the 5Gbit/s granularity creates a massive waste of resources in multi-granularity hard isolation services. To optimize the utilization of slot resources, we propose a new FlexE calendar slot allocation mechanism based on a novel hybrid-granularity model. This mechanism encompasses traffic prediction and a calendar slot allocation method based on a FlexE hybrid-granularity model. The former adapts the slot allocation process to the fluctuations in client flows through accurate traffic prediction. The latter adopts a hybrid-granularity slot allocation algorithm based on dynamic programming to ensure a high isolation of the service transmissions and to improve the utilization of slots. A comparison with the existing schemes shows that under experiments with different periodic regularities, the proposed method can increase the slot utilization by 71.5%-77.9%, and under experiments with diverse client granularity distributions, the proposed method can increase the slot utilization by 59.2%-76.7%.