原句是可以的,但是用从句不够简洁,如果用phenomenon,建议用被动式.
The phenomenon is witnessed that the economy has been booming in the past several years.
用phenomenon感觉有点生硬,以下几句供参考：
The past several years have witnessed the economy booming.（最简洁）
It is witnessed that the economy has been booming these years.