The ramp up stage of the steam chamber is the key stage of the dual horizontal well Steam-Assisted Gravity Drainage (SAGD) process. The rising speed, time and the corresponding oil rate are important indexes to be paid attention to in the ramp up stage of the SAGD process. The analytical model is an important theoretical method to guide the development of the SAGD process, which is conducive to the rapid design, evaluation and comprehensive adjustment of SAGD projects in field applications. However, a simple and accurate predictive analytical model for the ramp up stage of the steam chamber is still unavailable. In this study, initially the oil rate prediction model in the ramp up stage of SAGD of double horizontal wells is developed, according to the principle of mass conservation and the understanding of the development of the steam chamber, assuming that the slope angle in the ramp up stage of the steam chamber is a variable related to reservoir parameters and operational parameters, and considering the influence of anisotropy on the reservoir permeability. Then, the initial slope angle and the head availability factor were related to reservoir parameters and operational parameters by combining the numerical simulation method and the orthogonal test analysis method. In this way, the oil rate prediction model for the ramp up stage of the SAGD process was modified and improved. Furthermore, the new model was validated with Butler’s model and field data. The results show that the initial slope angle and the head availability factor of the steam chamber are not constants, but variables related to reservoir parameters and operating parameters. A larger initial slope angle will result in a shorter time required for the steam chamber to reach to the top of the reservoir pay zone, and a greater oil rate in the ramp up stage of the steam chamber. A greater reservoir permeability, a greater vertical to horizontal permeability ratio and a greater steam injection pressure will result in a greater initial slope angle and a greater instantaneous oil rate at the same time. The order of influence is: the vertical to horizontal permeability ratio > permeability > steam injection pressure. In addition, a greater reservoir thickness will lead to a smaller initial slope angle and a shorter time required for the steam chamber to reach to the top of the reservoir pay zone. The new model has the advantages of simple theory, fast calculation and accurate calculation results, which can provide theoretical guidance for rapid optimization of SAGD development.