I want to exercise to loose weight but everytime i do i got asthma attack after.

Time for consult: You need to consult with your doc.It is evident your asthma is not well controlled or you would not have an event after a workout. Being on a maintenance med and achieving better control should end the issue.

Pre-ex treatment: One effective treatment for exercised induced asthmas such as what you are describing, would be use a rescue inhaler pre-exercise. Alternatively, different exercise environments could be trialed (i.e. warm, cool, avoid chlorine/pools, etc). The caveat is that if you are having this many exacerbations, perhaps your maintenance asthma treatment needs to be adjusted.