"beyond my imagination" is correct and usable in written English. It is a phrase that means something is so extraordinary or unexpected that it surpasses what one can imagine. For example, "The performance was beyond my imagination and I could not believe what I had just seen.".
It was beyond my imagination".
Working abroad was enriching beyond my imagination.
I would be rich beyond my imagination.
But a person being burned alive is beyond my imagination.
"I was productive beyond my own imagination".
I stretched myself beyond my wildest imagination.
"The pace and kind of work that I do is beyond my wildest imagination," said Mr. Rajagopal, who is part of a worldwide team that develops graphics devices.
Being a terminologist, I care about word choice. Ludwig simply helps me pick the best words for any translation. Five stars!
Maria Pia Montoro
Terminologist and Q/A Analyst @ Translation Centre for the Bodies of the European Union