This paper deals with the construction and optimisation of accurate condition-based maintenance policies for cumulative deteriorating systems. In this context, the system condition behavior can be influenced by different environmental factors which contribute to increasing or reducing the degradation rate. The observed condition can deviate from the expected condition if the degradation model does not embrace these environmental factors. Moreover, if more information is available on the environment variations, the maintenance decision framework should take advantage of this new information and update the decision. The question is how shall we model the decision framework for this? A gamma process-degradation model with randomized parameters is proposed to model the influence of the random environment on the system behavior. An adaptive maintenance policy is constructed which takes into account the environmental changes. The mathematical framework is presented here and a numerical experiment is conducted to highlight the benefit of our approach.