Based on the broadly obtained experimental results, a particular simulant curve was firstly given to visually describe the variety of friction coefficient (μ) with the thickness of sulfide layer on hard steel substrate. The curve includes two inflexions named super-critical thickness (tC2) and sub-critical thickness (tC1) respectively. By means of “micro-peak and soft/hard plane” contact model given in the present work, physical definition and influence factors of tC1and tC2 were discussed in detailed. Theory analysis presents that a friction-reducing coating would obtain a minimum of μ at tC1，and good friction-reducing and anti-wear behaviors with its thickness range from tC1to tC2. Therefore, it is beneficial to improve tC2 or decrease tC1, especially keep a wide difference ( tC = tC2 - tC1). Herein, two optimized designation principles of the sulfide layer on a hard steel substrate were presented, which apparently apply to all lubricate situation with soft layers.