Autistic children show higher rates of co-occurring emotional and behavioural difficulties compared to other children in the general population. However, the environmental factors which contribute to the development of emotional (internalising) and behavioural (externalising) difficulties in autistic individuals are poorly understood. This study sought to investigate the association between harsh parenting (smacking, shouting, telling off) and the trajectories of emotional and behavioural difficulties in autistic children from ages 3 to 7. A sample comprising of 349 autistic children participated from the UK's Millennium Cohort Study. Associations between harsh parenting and child emotional and behavioural difficulties were modelled using multilevel growth curve models. In autistic children, harsh parenting was associated with total emotional and behavioural difficulties and behavioural (but not emotional) problems concurrently. Moreover, harsh parenting was not associated with changes in emotional and behavioural difficulties over time. Harsh parenting may have an important role in externalising problems in young autistic children indicating the need for public health strategies which educate parents on its effects.