A new steel-ultrathin UHPC lightweight composite deck (named as new LWCD for short) was proposed by using ultra-short headed studs in order to meet the demanding requirements in retrofitting and strengthening steel deck systems for long-span flexible bridges. The experimental tests were performed for the new LWCD via steel-ultrathin UHPC composite slab specimens, and the influence of key design parameters on the anti-cracking behavior of the specimens was analyzed. The test results show that the cracks widened approximately linearly with the increasing load when the maximum crack width was less than 0.15 mm. The maximum crack width in UHPC rapidly increased when the steel reinforcement yielded. The nominal cracking stress of UHPC was significantly affected by the reinforcement ratio and rebar diameter. Different methods of predicting the crack width in UHPC were compared based on the test results, and the proposed formula for calculating the crack width of steel-ultrathin UHPC composite slab was determined. Global and local finite element (FE) analyses were performed based on a long-span suspension bridge to validate the feasibility of the proposed new LWCD. The analysis results show that the self-weight of the new LWCD is comparable to that of the original 60 mm asphalt overlay. The internal forces in main cables and suspenders are increased less than 3.0%. The stress ranges in typical fatigue-prone details of the orthotropic steel deck (OSD) are reduced by 10.1%-52.0%, and the stress ranges in the OSD are all below the corresponding fatigue strengths (under 2 million cycles). The maximum tensile stress in UHPC caused by design loads was 8.4 MPa, much less than the nominal cracking strength obtained in the experimental test.