The ISIS Neutron Facility of Rutherford Appleton Laboratory (RAL) in the UK plays an important and world leading role in in-situ engineering materials testing, one of the most typical neutron diffractometers known as Engin-X, used to measure residual stress and phase transformation and to do micromechanics research, through using different sample environment equipment, such as mechanical fatigue loading frame, cryogenic temperature furnace of cooling the sample down to 1.5 K and particularly high temperature furnace of heating the sample up to 1100 ℃ under loading condition. The present maximum heating capability of the Engin-X high temperature furnace at ISIS can be increased to above 1100 ℃, that would allow more extremely challenging high temperature engineering problems around the world to be investigated. With this ambition in mind, in this paper we use TracePro software initially to optimize the geometry of the present Engin-X furnace reflectors and their configurations’ arrangement. One is to use ellipse-sphere combination and the other is to use ellipse-sphere-ellipse combination to replace the present Engin-X high temperature furnace’s half ellipse reflector geometry. The results show that the former plus further reflector surface coating and reasonable side shielding arrangement result in a total increase of 109% of energy absorption by the sample. The latter makes a further 6% of increase of energy absorption by the sample. Such results are further checked by subsequent ANSYS thermal analysis to investigate the temperature distributions within the centre portion of the sample. The ANSYS simulation results further reveal that both the ellipse-sphere and ellipse-sphere-ellipse configurations are able to increase the maximum capability of the Engin-X high temperature furnace at ISIS from the present 1100 ℃ to 1399 ℃ and 1423 ℃, respectively. In this paper, we present the details of the simulations and all the configurations of the Engin-X high temperature furnace.