High-power fiber lasers generate local heat load extremes during their operation, which increase the fiber temperature and lead to adverse thermal effects, such as transverse mode instability or cladding/coating thermal damage. The local temperature extremes are usually located near the end of a fiber where the pump power is delivered. In this paper, longitudinally inhomogeneous doping concentration profiles are applied to reduce the heat load extremes. Utilizing a new degree of freedom, it is shown by both simulations and measurements that maximal temperature along the fiber can be effectively decreased by using active fibers with an increasing concentration profile in the direction of pumping power. The concept is studied by a comprehensive numerical model that considers temperature-dependent parameters and is also demonstrated by measurement on an in-house built thulium-doped fiber laser formed by spliced sections with different concentrations. The output power of 54 W with the slope efficiency exceeding 62 % was reached.