I am using BDB to store my data,and associate several secondary DBs to
the primary DB,secondary DB uses btree as access method,now I want to
implement a simple select SQL function like this:
select * from flow where lasttime < 115075860 and lasttime >= 115075800
lasttime is a secondary key, How to accomplish this target in BDB?

You need to use a cursor and DB_SET_RANGE.

The following SQL contains two compares:
select * from flow where lasttime < 115075860 and lasttime >=
115075800.

It seems that DB_SET_RANGE only fetchs the record that is the smallest
key greater than or equal to the specified key.
Does it mean that it only return a SINGLE record and only implement
just ONLY ONE compare?
Then I need to use cursor and DB_SET_RANGE to return mutiple data sets
and then extract these records and to implement left compares in other
programm language(C,java,etc) manually?

Yes.

Yes, you need to iterate over key/data pairs until you find one whose
key violates the second part of the condition.