The measures include increasing the scope of eligible products and increasing quotas.
The fall in AUM occurred despite net fund inflows of HK$88bn in Hong Kong last year.
According to the joint circular, intermediaries may apply a streamlined approach when dealing with SPIs.
The adjustments will be implemented in about three months time.
There is no evidence of losses for customers.
The city’s assets under management (AUM) in asset and wealth management only reported a 2% year-over-year increase for 2021, official figures show.