Can I copy the "background" and "related work" part of my published paper? For example, I want to propose a new algorithm for an existing problem, and I already have published several papers about this topic, they share exactly same background, is it OK for me to copy the "background" and "related work" part from my published paper?

The background contains some already well-studied techniques, in order for the paper to be self-containing, they should be briefly introduced (instead of just being cited). 

I don't see the meaning that I have to express the exact same thing in a different way for every new paper
This is called "text recycling" or even "self-plagiarism" and generally frowned upon by editors. It's better to avoid it as much as possible, even if it results in more work, particularly for non-native speakers of English. Too much overlap can result in rejection or later retraction, where "too much" is at the discretion of the editor. You are also very likely to annoy reviewers if they notice the overlap with your previous publications.

The Committee on Publication Ethics has the following guidelines for journal editors:


  When should action be considered?
  
  Text recycling can take many forms, and editors should consider which
  parts of the text have been recycled. Duplication of data is likely to
  always be considered serious (and should be dealt with according to
  the COPE guidelines for duplicate publications [1,2]. Use of similar
  or identical phrases in methods sections where there are limited ways
  to describe a common method, however, is not uncommon. In such cases,
  an element of text recycling is likely to be unavoidable in further
  publications using the same method. Editors should use their
  discretion when deciding how much overlap of methods text is
  acceptable, considering factors such as whether authors have been
  transparent and stated that the methods have already been described in
  detail elsewhere and provided a citation. Duplication of background
  ideas in the introduction may be considered less significant than
  duplication of the hypothesis, discussion, or conclusions.
  
  When significant overlap is identified between two or more articles,
  editors should consider taking action. Several factors may need to be
  taken into account when deciding whether the overlap is considered
  significant.
  
  Text recycling in a submitted manuscript
  
  Text recycling may be identified in a submitted article by editors or
  reviewers, or by the use of plagiarism detection software, e.g.
  CrossCheck. Editors should consider the extent of the overlap when
  deciding how to act. Where overlap is considered to be minor, authors
  may be asked to re-write overlapping sections, and cite their previous
  article(s). More significant overlap may result in rejection of the
  manuscript. Where the overlap includes data, Editors should handle
  cases according to the COPE flowchart for dealing with suspected
  redundant publication in a submitted manuscript [1].